The Art of Reading Buildings PDF

ESSENTIAL BUILDING CONCEPTS OBJECTIVES Define loads and imposition of loads as they relate to buildings. List three types of forces created from imposed loads. Explain the effects of fire on common building materials. List the factors that determine the suitabi...

ESSENTIAL BUILDING CONCEPTS OBJECTIVES Define loads and imposition of loads as they relate to buildings. List three types of forces created from imposed loads. Explain the effects of fire on common building materials. List the factors that determine the suitability of a building material for a given application. Describe the relationship of surface-to-mass ratio and fire degradation for building materials. Differentiate the terms native wood, traditional wood products, and engineered wood products. WHY SHOULD YOU READ THIS CHAPTER? he fire officers who read this chapter are better prepared to not only "read" a building, but better prepared to communicate building proble s, hazards, and collapse potential. Likewise, fire officers need to learn a certain language that is used in the building construction/engineering field. That language helps the fire officers teach new firefighters, prepare more complete prefire plans, and interact with building representatives and engineers during incidents. The intent here is not necessarily to make you a building engineer-far from it. We do, however, define basic terms and concepts that come from the building construction world and help provide a bridge to the 2:00 a.m. fire suppression world. Additionally, this chapter helps address an evolving issue that is related to younger generations of firefighters. The fire service is experiencing an evolu­ tionary change in the types of individuals who serve as firefighters. In the 1960s and 1970s it was very common for firemen (as they were called) to possess a significant understanding of building construction trade knowledge. They were 11  The Art of Reading Buildings carpenters, plumbers, electricians, roofers, and with simple definitions, then adds some specificity masons as their primary career (volunteers) or as for certain applications and situations. part-timers on their off-duty days (career). Those who didn't possess that experience/knowledge often Dead loads. The weight of the building itself and learned building construction by helping other anything permanently attached to the building. firefighters with home projects, trading favors, Live loads. Any load applied to a building other firehouse do-it-yourself projects, and such. The than dead loads. Live loads are typically transient, bottom line-most fire training programs didn't moving, impacting, or static (like furniture). have to emphasize building construction because it was a common knowledge base. Unfortunately, Live and dead loads can be further classified as that is not true today. to their application nature. For example: While skilled building tradespeople still exist Concentrated load. A load that is applied within a in the fire service, their numbers are dwindling small area or at one point. quickly. Many of today's new recruits have little Distributed load. A load spread over a large sur­ building trade background. Terms such as ledger, face area or over multiple points. torsion, linteL silL ductile, and rafter tie plate have little meaning to them, and they are unable to trans­ Impact load. A moving or sudden load applied to late these terms into determining strengths and a building in a focused or short time interval. For weaknesses when reading a building. Thankfully, example, wind, large crowds, and fire stream water these new recruits are well versed in gigabytes, are all impact loads. phone apps, extreme sports, and multitasking­ which show they have a tremendous capacity to Repeated load. Loads that are transient or inter­ learn and absorb knowledge. mittently applied (like people on an escalator). This chapter outlines some essential terms and Static load. A constant load that rarely moves. concepts as it relates to loads, forces, and materials Suspended load. A load that is hanging from that are used to assemble the building that may something above. ultimately fail/collapse during fire suppres­ sion operations. Wind/snow load. Atmospheric loads that stress a building. The fire service uses the term fire load, which is LOADS the potential amount of heat energy (measured in British thermal units-BTUs) that may be released when a material is burning. The term fire load is To be sound, a building must be designed, not a building engineering term-it's purely a fire engineered, or otherwise assembled to resist a load. service one. Loads are static and dynamic weights that come from the building itself and anything that is placed within or acts upon a building. Gravity is respon­ sible for creating most loads-as any natural or Load imposition human-created thing that has weight is being pulled A firefighter team making a panel cut on a to the planet's surface by gravity. Likewise, gravity pitched wood roof for heat ventilation (while is trying to flatten buildings-whether the building working from a roof ladder) can be classified as a is on fire or not. Other loads come from atmospheric live, impact, and distributed load (the firefighters conditions such as wind, rain, and snow. To help are the live load, the movement of the firefighters classify all these loads, the building industry starts are the impact load, and the ladder beams are 12 Chapter 2 ESSENTIAL BUILDING CONCEPTS n distributing the firefighters' weight as a distributed load). That live, impact, and distributed load is :u ;mposed on the roof and its supporting elements. The imposition of loads refers to the contact orien­ n tation of the load to the material(s). Loads can be imposed three ways (fig. 2-1): ii Axial load. Load is imposed through the center of the material. t... ILi A. B. C. Eccentric load. Load is imposed off-center, caus­ ing a material to want to bend. Fig. 2-2. There are three forces created when loads are Torsion load. Load is imposed in such a way that imposed on materials: (A) compression, (B) tension, and causes a material to twist. (C) shear. Obviously, the material chosen to receive the imposed load must have characteristics that resist the forces that are created within it. Further, the material must transfer that load and eventually deliver the load to earth in compression. Note: There are very few building/structure applications that deliver a load to earth in any force other than compression. Guide (guy) wire anchors that are drilled deep into the earth for a suspen­ AXIAL ECCENTRIC TORSION sion bridge or tower antennas are noted exceptions (Thru Center) (Off Center) (Twisting) (they are delivered in tension and/or shear). Next, Fig. 2-1. Loads can be imposed three ways: axially, eccentri­ we discuss the relationship between loads and cally, and torsionally. material characteristics. Quick summary Forces resisting loads Loads are static and dynamic weights that exist Imposition of loads on a given material causes within or on a building. stress within the receiving material. These stresses There are basically two loads that exist within are called forces. Forces help resist the load. There every building: live and dead loads. Live and are three primary forces created within materials dead loads can be further classified as impact, (fig. 2 2): - suspended, distributed, and concentrated. Compression. A stress that causes a material to Fire load is primarily a fire service concept. flatten or crush. Loads are imposed three ways: axially, eccentri­ Tension. A stress that causes a material to pull cally, and torsionally. apart or stretch. Imposed loads create forces within materials: Shear. A stress that causes a material to tear or compression, tension, and shear. slide apart. 13  The Art of Reading Buildings BUILDING MATERIAL From a fire-fighter's point of view, the more mass a material has in a given surface area, the more time CHARACTERISTICS (or heat) is required before the material starts to degrade. When a material does degrade it may also deform-thus changing its engineered shape and The previously mentioned firefighters making the way loads are resisted. A change in a material's the ventilation panel cut on the pitched roof are a shape causes a change in load imposition-poten­ live, impact, and distributed load being imposed tially resulting in rapid failure (fig. 2 3). - in an eccentric manner that is creating a compres­ sive force on the roofing materials that the ladder beams are resting upon. If you can picture this in your mind's eye, then you are well on your way to reading buildings. This whole load/imposition/force equation relies on certain material characteristics to prevent failure. When discussing the characteristics of materials, we have to understand that there are many factors that determine the suitability of the material for a given application. These factors include the following: Fig. 2-3. A change in load imposition can result in Type of material (wood, steel, concrete, etc.) rapid failure. Shape of the material (round, square, rectan­ gular, etc.) The building engineering community also Orientation or plane of the material (vertical, considers mass, but from a different perspec­ horizontal, etc.) tive. Economics, sustainable-resource concerns, and technological advances have challenged the Mass of the material (surface-to-mass ratio, building community to maximize strength with density, depth, etc.) reduced material mass. Basically, most new build­ Material surface (rough, slippery, hard, or ings are high-strength/low-mass buildings. If mass adhesion/connection ability) is time during a structure fire, then time has been lost to fight fires in new buildings with modern Additionally, the fire service looks at how lightweight construction methods and materials. these materials react during a fire and how their Granted, the building may be stronger, but that ability to resist a load changes during fire condi­ assumes that building materials are not being tions. Of importance to firefighters is the concept heated, have not changed shape, and are loaded of surface-to-mass ratio. The more mass a material within their engineered design limits. has relative to its exposed surface area, the more it is resistant to heat. We learned in fire behavior The engineering community also classifies class the concept of heat flow: Heat flows from hot materials as being brittle or ductile based on their to cold (heat seeks cold). Materials exposed to the reaction to imposed loads and resistive forces. radiant heat of other burning materials serve as a heat sink. The amount of heat the material can Brittle. A material that will fracture or fail as it is absorb before it starts breaking down is directly deformed or stressed. proportional to its surface-to-mass ratio. In essence, Ductile. A material that will bend, deflect, or stretch mass is heat resistance, and heat resistance is time. as a load is applied-yet retain some strength. 14 Chapter 2 ESSENTIAL BUILDING CONCEPTS Simply stated, a brittle material breaks before previously been used as structural members in it bends and a ductile material bends before it building construction. breaks. Wood, plastics, and most metals are ductile, whereas masonry, tile, and cast iron are brittle. With an understanding of building characteristics, Native wood (cut or sawn lumber) we can now discuss individual materials. ative woods (or sawn lumber)-whole lumber piefes cut from a tree-are not created equal. Th re are hard woods, soft woods, tight grained, Quick summary kn t ty, old-grow th, and new-growth woods. Factors that can help materials to work or fail Ea¢h has interesting characteristics and the true can be defined as type, shape, orientation, craftsman woodworker knows how to maximize mass, and surface. the! strength and application of each. However, it woijild be easy to assume that wood taken from the The surface-to-mass ratio of a material is tre s of today is roughly the same as wood that was especially important to firefighters. Mass is heat tak n from trees yesterday. Unfortunately, that is resistance. Loss of mass (high surface-to-mass notlthe case as it is relatively easy to overlook the ratio) means loss of time during fires. fact that the wood currently being used in modern Most new buildings are comprised of high­ building construction is significantly different from strength/low-mass materials such as light­ the wood that was used in older construction. From weight wood construction. a logging perspective, old-growth trees that were common a hundred years ago are just a memory, Materials can also be classified as either brittle with new-growth trees (or second-growth trees) and (breaks before bending) or ductile (bends before plantation trees normally replacing the older trees. it breaks). Interestingly, it is common for today's timber indµstry to harvest trees similar to corn or wheat. As 1an example, pine and spruce trees can often SPECIFIC BUILDING be ut 25 years after they are planted. This is why mol:lern lumber trucks routinely carry numerous MATERIALS sm ller logs (fig. 2-4) instead of y esterday's logging trucks that routinely carried several In the past, four basic materials were used to larie logs. This has resulted in wood that is not erect buildings: wood, steel, concrete, and masonry. only different, but wood that also burns signifi­ Today, advanced material technologies have created can;tly hotter and faster. Old-growth trees produce composites of the aforementioned materials as well a ood that is denser and has a reduced level of as new plastics, graphite, wood derivatives, and pit h (which burns like a petroleum-based product). exotic metals. Let's look at the four basic building Ad itionally, in the past it was not uncommon for materials (wood, steel, concrete, and masonry) as wo d-particularly wood that was to be used for str ctural members-to be cut from the heart of I well as some of the new composites, particularly wood composites (often referred to as engineered a tree (which has maximum density and minimal wood). In this section, we take a closer look at wood pitch). Conversely, new-growth trees are less dense as it is the most predominant building material of and have a higher concentration of pitch. This has the past, present, and likely the future. Engineered resulted in wood that is lighter in weight (which can wood products are not only progressively replacing reduce strength), is capable of burning more rapidly native wood, but also concrete and steel that have and with a higher British thermal unit (Btu) heat l 15  The Art of Reading Buildings level, and in cheaper types of wood being used for In the past, firefighters could gauge when a solid structural members. hardwood beam would collapse because it started to sag a bit before it failed. Today, lightweight wood construction can catastrophically fail in a short period of time (compared to conventional lumber) and without prior warning signs such as sagging. Traditional wood products Used here, the phrase traditional wood products refers to the century-old development and improve­ ment of manufactured wood products for a specific application that cut lumber cannot fill. Namely, we are talking about heavy timber, glue-laminated Fig. 2-4. New-growth logs are typically smaller in diameter beams/columns and sheathing. Within this context, and length and less dense than old-growth logs. engineered wood refers to modern, technologically advanced wood products and is covered separately after this section. As a result, it should not be surprising that around 1986 the lumber industry changed its Glued laminated heavy timber. Originally, the rating system from Utility (utl), Standard (std), common availability of large trees meant that large Construction grade (cons), and Select (sel), to #3, pieces of sawn lumber were available for use in #2, and #1 (best). Additionally, Douglas fir was the sizeable buildings that required large supporting standard for exterior bearing walls (the good stuff), and/or structural members (this is readily observ­ with white fir or hemlock used for interior walls able in large older buildings). As large trees began (the cheaper stuff). Today, the building construc­ to become more rare, the original glued laminated tion industry routinely uses the aforementioned beam or column (glulam for short) was born from cheaper grade of lumber for exterior load-bearing the fact that trees only grow so wide and so thick. walls. To compound the problem, today's sawn It was quickly discovered that a heavy timber beam wood is known as nominal dimension lumber as could be created by using a number of smaller cut opposed to yesteryear's full-dimensional lumber. lumber pieces (fig. 2 5). - Previously, a 2 x 4 was really 2 in. by 4 in. A nominal dimension 2 x 4 is 1½ in. by 3½ in. When the preceding factors are combined, it should not be a surprise that modern structures built with this type of lumber (that is used to replace conventional construction and also used in various lightweight configurations) are collapsing faster and burning hotter than the structure fires of yesterday. Despite this fact, while from an engineering point of view wood has marginal resistance to forces compared to its weight, it does the job and is the most used building material. We also know that wood burns and when it does, mass is lost. T he more mass a section of wood has, the more Fig. 2-5. Glulams are a popular modern replacement for solid lumber. material must burn away before its strength is lost. 16 Chapter 2 ESSENTIAL BUILDING CONCEPTS These pieces were originally strapped or bolted plywood based on its grain density, thickness, together before suitable glues were developed. As and gluing/coating process. When exposed to described here, glulams are heavy timbers and fire, the plywood layers start to char and burn can absorb lots of heat prior to failure. They also away, layer by layer. When exposed to serious burn forever! However, under heavy (or lengthy) heat (as opposed to flame), the layers dry out fire conditions, glulams can fail and often cause and begin to curl. The destruction is usually a failure of large sections of a building (fig. 2-6). easy to detect. Obviously, the thicker the The glues are deeply impregnated and protected plywood sheet, the longer it can withstand heat by the shear mass of the wood pieces used. and flame, but similar to other wood products However, remember that the glues or adhesives that use adhesive compounds for bonding can, depending on their chemical composition, emit strength, a toxic smoke can be emitted under toxic gases when burned or exposed to heat. fire conditions. Generally speaking, plywood has been replaced with a true engineered wood product-oriented strand board (OSB)-which is covered in the following section. Particle board. Wood sheathing made from a coarse sawdust and glue is known as particle board (PB). Particle board appears very smooth and consistent and has no wood grain. The sheathing is relatively heavy due to the compac­ tion of the sawdust and glue during manufac­ turing. Even with this density, PB sheathing is actually quite weak with low resistance to trauma (it cracks and crumbles easily). In Fig. 2-6. When used as primary structural members, the failure fact, its own weight can cause a large sheet to of glulams can cause failure of large sections of a building. crack just from flexing. Because of its fragility, PB used as a floor or roof cover must be well supported with closely-spaced joists (beams) Sheathing. We can think of sheathing as merely a and include a durable surface covering. Fire cover for something. While floor and roof sheathing and heat will easily destroy particle board. PB needs to have some strength, wall sheathing breaks down so easily in heat conditions that it requires very little. This led to the development of is the sheathing of choice for the pyrolyizing traditional wood sheathing products to maximize (off-gassing) fuel source in flashover simula­ the waste parts of a tree left when lumber was tors. The smoke produced from the degrada­ cut. All sheathing products can be considered tion of PB is full of wood particles and sticky high surface-to-mass. Traditional wood sheathing aerosols (hydrocarbon-based glues) that are includes plywood, particle board, and decora­ quite flammable and toxic. tive (paneling). Decorative sheathing. Thin wood paneling Plywood. Often called the original engineered used to finish interior walls or the outside of wood product, plywood is made from layering cabinets are classic examples of decorative sheet veneers of wood such that grain directions sheathing. These products are not intended to alternate 90° with each layer. (This is similar to resist loads and are merely decorative. This the engineered wood products that are listed in sheathing can range in thickness from ½ in. to the following section.) These layers are glued ¾ in.-meaning a high surface-to-mass ratio. to each other as they layer together. There are Because of the rapid flame spread character­ various grades of strengths and applications of istic of decorative wood paneling, most are 17  The Art of Reading Buildings not allowed by code for interior wall finishing. and is amazingly lightweight compared to natural Additionally, if adhesives are used (which is forested woods. likely), they will emit toxic gases under fire conditions. Once harvested, the wood is milled into veneers, wood chips/slivers, or shavings (shredded wood fiber or pulp). The milled product is then processed Quick summary into forms, emulsified in binding agents (glues/ adhesives), then autoclaved (application of heat and Old-growth trees were widely used for large pressure) to set the binding agent. The glues that timber structural members in older buildings. bind engineered wood products require only heat These members can resist the effects of fire to break down and are also toxic, combustible, and for longer time frames than newer construction will contribute to burning. materials.due to mass. Engineered wood products are currently Newer lumber is typically harvested from being used as a replacement for solid sawn wood new-growth trees, which results in a softer materials (cut lumber) in common applications due wood with a higher pitch content. to their advertised advantages of higher strength; Older conventional roofs can often sag before greater stability over longer spans; resistance to collapsing, while newer lightweight wood shrinking, crowning, twisting, and warping; ease trusses do not prior to. catastrophic collapses. of manipulation; and efficiency in using more Traditional wood products refer to older portions of a tree. Following are common examples sawn lumber products like glulams and of engineered wood products that rely on adhesives plywood sheathing. for bonding strength. Plywood, particle board, and decorative Oriented strand board (OSB). Known mostly by sheathing use adhesives that may emit its acronym, OSB is sheathing that is formed with flammable and toxic gases when exposed wood shavings and a urea-formaldehyde adhesive. to heat or fire conditions. The wood chips are oriented such that the grain directions are randomly oriented and layered. An adhesive locks these layers in place such that Engineered wood products multidirectional and uniform strength is achieved. OSB is used extensively in new construction as a While no official definition exists, the term structural sheathing to form roof and floor assem­ engineered wood is used by the fire service to blies (when glued to trusses) and as the web portion describe a host of wood products that use modern of a wooden I-beam (fig. 2 7). - OSB is subject to methods to transform wood chips/slivers, veneers, degradation by direct sunlight (UV rays), moisture, shavings, and even recycled wood products into and heat. The heat of fire or smoke can cause rapid components that replace sawn lumber, sheathing, destruction of OSB. Likewise, direct flame contact and other composite structural materials. The wood will cause OSB to ignite and burn rapidly and emit used to make engineered wood products (EWP) toxic gases from adhesives. is typically derived from new-growth forests and The strength and economy of OSB has led to a rapid-growth tree farms, although in some cases proliferation of its use in residential buildings. As it is possible to manufacture similar engineered an example, a typical 1,800-sq-ft two-story new cellulosic products from other lignin-containing home would use a concrete foundation, I-beams for materials such as hemp stalks, wheat straw, and floor joists (that use OSB for the webbing), OSB other vegetable fibers. In either case, the wood sheathing for the flooring, OSB I-beams for second used for EWP is loose grained, has lots of pitch, floor joists, OSB stair treads and kickers/risers, 18 Chapter 2 ESSENTIAL BUILDING CONCEPTS exterior OSB sheathing, and OSB roof sheathing attached to trusses that may also include OSB stiffeners. As a result, a major portion of this home would be comprised of wood chips that are bonded with an adhesive. Fig. 2-8. Laminated veneer lumber is often used in place of cut lumber. Laminated strand lumber (LSL). LSL is a struc­ tural composite lumber manufactured from flaked and chipped strands of native wood blended with an adhesive. Mostly, LSL uses strands oriented in a parallel fashion (also known as parallel strand lumber-PSL). PSL is similar to LVL in its use. Fig. 2-7. OSB is used for sheathing and for the web The primary difference is that LVL uses sheet of engineered wooden I-beams. veneers of native wood whereas PSL uses flaked wood strands. The glue may be phenolic resin, urea-formaldehyde, or phenol formaldehyde. All Laminated veneer lumber (LVL). To form LVL, of these glues are derived from crude oil. Fire thin sheet veneers of native wood are stacked with behavior wisdom suggests that LSL/PSL will grains aligned and then glued with a phenolic fail before LVL. LSL/PSL can be used as beams, resin. LVL is used in place of cut lumber for beams headers, studs, and rim boards. (fig. 2-8). LVL is also used to form the chords (or flanges) that are glued to the OSB web of Cross-laminated timber (CLT). CLT is an engi­ engineered wooden I-beams. LVL is designed to neered wood product using several layers (three to have the load imposed axially and perpendicular seven or more) boards that are layered crosswise to the grain. While the mass of LVL is typically (typically rotated 90 ° ) and glued. CLT is used as a higher than OSB, it is still degraded by the heat of structural element for columns (much like a glulam a fire or smoke. is used for beams). CLT uses actual timber boards cut from smaller trees to form a panel and the Because LVL is formed with native wood crosswise layers. In many ways, CLT is a structur­ veneers, the individual sheets hold together until ally sound form of plywood but thicker. CLT can the wood burns. The glue that binds each layer also be used for long spans and structural assem­ tends to cause delamination of the veneer sheet blies that are used for roofs, walls, and floors. by sheet when heated. LVL is commonly used as a replacement for conventional sawn lumber and This product is gaining widespread acceptance timber for beams, joists, rafters, columns, studs, l due to its improved acoustics over sawn lumber and rim boards. and its reduced carbon emission footprint. For example, for every 1 ton of wood, it takes 5 times more energy to produce 1 ton of concrete, 24 times 19  The Art of Reading Buildings more energy for 1 ton of steel, and 126 times more The proliferation of engineered wood products energy for 1 ton of aluminum. CLT also advertises a over solid wood is based on multiple advantages greater resistance to fire. This claim is based on the that include greater strength and stiffness, pound premise that due to the solid nature of the material, for pound strength that is greater than steel, more it will char at a slow and predictable rate (similar efficient use of wood (use of smaller pieces, wood to mill construction type members). The char on with defects, wood chips, etc.), and conformance wood forms a crust that slows the burning rate and to emerging "green" considerations. From a fire helps shield the wood from further degradation. service perspective, however, there are huge disad­ vantages associated with the use of EWP. Glued laminated timber (GLT). GLT is comprised of multiple layers of dimensional timber bonded Some products may burn faster than solid together with moisture-resistant adhesives. GLT lumber; they have a high surface-to-mass ratio. is a more modern form of the traditional glulam Some adhesives are toxic, and some resins heavy timber covered above. GLT can be used as can release formaldehyde (urea-formaldehyde horizontal beams and vertical columns, and can resins). Currently, there are four basic types also be produced in curved shapes, which makes of adhesives that are used in engineered wood this product very attractive to interior designers products: urea-formaldehyde (most common), who want visible structural members that are more phenol-formaldehyde, melamine-formaldehyde, decorative than straight members. and methylene diphenyl diisocyanate (expen­ Finger-jointed lumber (FJL). FLJ has become a sive). So, it is easy to see that three out of the common method to produce long lengths of wood four adhesives are formaldehyde based. members from multiple short pieces of native As adhesives pyrolize, the resulting gases can wood lumber. When joining these short pieces, become flammable. the joining ends are mitered in an interlocking fingers configuration and pressed together with an adhesive as a bonding agent. Using the FJL process, Quick summary wood manufacturers can create a long, straight, and Engineered wood products are those made solid wood joist or stud from a bunch of scrap mill from wood chips/slivers, veneers, shavings, and ends. FJL can also be used to join one section of recycled wood that have been bonded using lumber to another (such as a 90 ° angle, as shown various adhesive methods. in figure 2-9) with an adhesive that is used as a bonding agent. The wood used for EWP is often harvested from rapid-growth tree farms. These trees are then milled (shredded into a pulp). The pulp is very lightweight, loose grained, and contains lots of pitch. OSB is the most prolific of the EWPs. It is used extensively in modern residential construc- tion for beams, structural sheathing, and stair assemblies. OSB is very susceptible to heat degradation and burning in fire conditions­ leading to rapid failure. Although engineered lumber can offer many advantages over sawn lumber, its primary Fig. 2-9. Finger-jointed lumber is commonly used to join one disadvantage to the fire service is threefold. section of lumber to another. --------- 20 Chapter 2 ESSENTIAL BUILDING CONCEPTS Steel has excellent resistance to compression, It has already begun to change the way build­ tension, and shear forces. Its strength-to-mass ings are being constructed when steel and ratio is excellent. Additionally, steel has factory concrete are replaced by wood products (see versatility; that is, it's relatively easy to fabri­ chapter 6), some applications can burn faster cate different shapes, sizes, and strengths during and with more intensity than sawn lumber (OSB production. For this reason, steel is a popular as an example), and many of the adhesives choice for large commercial structures. From a fire that are used as a bonding agent will emit toxic service viewpoint, steel has two weaknesses: it is gases such as formaldehyde. engineered for very specific applications (thick­ ness, length, shape, and strength) and it softens and elongates when heated. Steel In a fire, steel acts as a collector of heat-it con­ Steel has been a staple building material for ducts heat readily. Steel loses strength as tempera­ commercial buildings for almost two centuries. tures increase; the specific range of temperatures It is used extensively for columns and beams (the at which it loses strength depends on how the steel true bones of a building), especially in applications was manufactured. As a general rule, cold-drawn where strength, long spans, or tall walls are needed. steel like cables, bolts, rebar, and lightweight fas­ The classic I-beam and H-column are most associ­ teners loses 55% of its strength at 800°F. Hot-rolled ated with steel. More recently, lightweight steel structural steel used for beams and columns loses C-channel is being used to replace wood studs in 50% of its strength at l ,100 ° F. Structural steel occupancies that have noncombustible or low dead also elongates or expands as temperatures rise. load requirements. At l ,000° F, a 100 ft beam can elongate 10 in. In cases where a steel beam is affixed at both ends and Steel is made from iron ore, carbon, and an alloy heated, the steel can't elongate, thus it will twist, agent (metallic solid solution). During manufac­ sag, or buckle as it tries to expand. This deforma­ turing, iron ore is crushed and made molten using tion can cause an immediate and often general col­ a blast furnace and smelted with coke (a carbon lapse of floors and roofs. source that is a derivative of coal). Alloying agents ( like chromium, nickel, etc.) are added to help achieve strength and ductility. The molten solution Cast iron can then be formed into pieces by casting, hot rolling, or cold rolling. Casting is just that-the Cast iron is a material usually formed from pig molten steel is poured into a desired mold to form iron, which is a high carbon content iron. The iron the finished product. Hot-rolled steel is the result is heated until it liquefies and is then poured into of molten steel shaped at temperatures above the molds to solidify into desired shapes. Because of crystallization stage, which allows thinner sheets the high carbon content and lack of alloys, cast or shapes. Hot-rolled steel is often called extruded iron is brittle. Cast iron has good compressive steel. I-beams and H-columns are typically strength qualities and acceptable shear strength if hot-rolled extruded. Cold-rolled steel is shaped the cast iron was formed with significant mass. In as it cools ( below crystallization temperatures, the 1800s and early 1900s, cast iron was used in forming stronger steel). Some call cold-rolled steel structural applications such as columns and door/ cut or rolled steel. Nuts, bolts, cables, rebar, and window frames. Many historical buildings still wires are examples of cold-rolled steel. Lightweight have gorgeous, ornate storefront cast iron columns C-channel studs are also cold-rolled steel. (fig. 2-10). 21  The Art of Reading Buildings ---------------------------------------------- As a building material, aluminum is considered a soft metal that is high strength-to-weight, highly ductile, noncorrosive with air/water, and nonmag­ netic. Because of production costs, aluminum is rarely used for the main structure of a building but is used extensively for trim, brackets, finishes, sheeting, and special applications where light­ weight, noncorrosive materials are needed. During fires, the low mass and ductile nature of aluminum causes rapid failure. Titanium. Like aluminum, titanium is an abundant Fig. 2-10. Cast iron columns were used on the storefronts of metal found in many minerals and is lightweight, many older commercial buildings. low density, noncorrosive, and nonmagnetic. Titanium alloys are known for a high strength­ Unfortunately, cast iron is brittle and history has to-weight ratio and tremendous resistance to heat. shown that it can crack from aging, eccentric and/or Most firefighters consider "lightweight" as a recipe torsional loading, and trauma. Cast iron has great for rapid failure. Titanium is an exception to the resistance to slow heating and cooling. In a hostile rule. For most buildings, titanium is too expen­ fire environment, cast iron can initially resist heat sive to be used as a building material although its fairly well but tends to fracture (or crumble) more light weight and high strength makes it ideal for easily when an eccentric load is applied (sagging of innovative architectural designs (soaring beams, a floor or being struck with a powerful fire stream). twisting columns, etc.). As material technologies For this reason, cast iron is no longer used for progress, more variants of titanium will be found structural applications. in building materials. Research shows that there is still debate on whether the application of cold water to fire-heated cast iron causes explosive fracturing1. The debate Quick summary centers on two arguments: Steel has been a staple of the building industry for many years, and is a structural material The rapid cooling causes sudden contraction that is used for both lightweight studs {interior that implodes the brittle material. and exterior) and lightweight roof structural The physical force of the fire stream is an members as well as primary structural members impact load that rapidly separates the brittle in the form of I-beams and H-beams. material. Steel is made from iron ore, carbon, and an Regardless, most agree that cast iron is a brittle alloy agent that provides increased strength and material that can fail more easily when heated by fire. ductility. Steel products are either hot-rolled (extruded), cold-rolled (cut steel), or cast. Other metals Steel has excellent properties such as versatility and resistance to tension, compression, and Aluminum. Aluminum is a natural element that shear forces. exists in many minerals and ores. In fact, alu­ Steel will act as a collector of heat in a fire minum is the most abundant metal that exists on and lose much of its strength above 800° F, earth. Unfortunately, the reactivity of aluminum which can cause it to elongate, twist, and requires massive amounts of energy and refine­ ultimately fail. ment to produce the beer cans we are familiar with. 22 Chapter 2 ESSENTIAL BUILDING CONCEPTS I I material and then tensioned, compressing the I Cast iron has been used in the building industry concrete to give it the required strength. Cables can since the 1800s; however, as opposed to steel, be pre-tensioned (at a factory) or post-tensioned (at it is a brittle material. the job site). Cast iron can fracture when heated in a fire Precast concrete. Slabs of reinforced concrete that and then exposed to water from suppres­ are poured at a factory and then shipped to a job sion operations. site (fig. 2-11). Precast concrete can be used for Aluminum and titanium are abundant minerals walls, f loors, or roofs. Common applications of that have excellent strength-to-weight ratios. precast concrete are the venerable tilt-up slabs that Due to manufacturing costs, neither has been are used for walls and twin-T slabs used for floors used for the structural bones of a building and roofs. although that is starting to change. Aluminum fails quickly during fires whereas titanium shows significant resistance to heat. Concrete Concrete is a mixture of Portland cement, sand and aggregate (usually gravel), and water that cures into a solid mass. The curing process creates a chemical reaction that bonds the mixture to achieve strength. During the mixing process, gravel can be added as a volume and strength expander. The final strength of concrete depends on the ratio of these materials-especially the ratio of water to Portland cement. Low-slump concrete is stronger and has a lower water-to-cement ratio, while high-slump concrete is wetter and flows easier. Cured concrete has excellent compressive strength but poor tensile and shear strength. Pure concrete is considered a brittle material. For this reason, steel is often added to concrete as reinforce­ ment when the concrete is being used in a way that will subject it to those forces (like a floor or roof). When steel is added to concrete, the finished material is considered a composite-brittle with Fig. 2-11. Precast reinforced concrete slabs are being trans­ some ductile properties. Steel can be added to ported to a job site. concrete in many ways during its casting: Reinforced concrete. Concrete that is poured over Monolithic buildings are concrete buildings built steel rebar, which becomes part of the cured on location using a steel rebar frame and wood or concrete mass. composite material forms to shape the concrete. Concrete is then pumped into the forms encasing Pre- and post-tensioned concrete. Concrete that the steel-creating a reinforced concrete building. has steel cables placed through the plane of the Monolithic is derived from the Latin word for single 23  The Art of Reading Buildings stone. Monolithic buildings are typically built one concrete mass, causing catastrophic spalling and floor at a time. The columns are built ahead of the failure of the structural element. Concrete can also floors and utilize a slip form, which moves slowly stay hot long after the fir is out, causing additional upward as each level is poured (fig. 2-12). Floors thermal stress to firefighters performing overhaul. are then anchored into cured columns. The floors As an example, these conditions were present in the are built upon a scaffold-like platform (called false­ Central Library fire in Los Angeles, as described work). Once a floor cures, the falsework is removed in chapter 4. and rebuilt on the next level. Masonry Masonry is a common term that refers to brittle materials like brick, tile, concrete block, and stone. The classic concrete masonry unit (CMU )-some call it a cinder block-is the most common material used for building a masonry wall (fig. 2-13). Masonry is used to form load-bearing walls because of its compressive strength, but it can also be used to build a veneer wall (one that bears only its own weight). Individual masonry units are held together using mortar. Mortar is a workable paste made from a mixture of sand, cement or lime, and water. Once cured, the mortar serves as a binding agent for masonry blocks. These mixes have little to no tensile or shear strength; they rely on compressive forces to give the masonry strength. It is impor­ tant for firefighters to know that a masonry wall actually gets stronger as axial loads are applied and compressive forces increase. Obviously, there is an absolute maximum weight that can be applied before the brittle material fails. Fig. 2-12. A wooden slip form is used as a concrete mold for the construction of a monolithic building. Unlike steel, concrete is a heat sink and tends to slowly absorb and retain heat rather than conduct it. This heat is not easily reduced. All concrete contains some moisture and continues to absorb and wick moisture (humidity) as it ages. When heated, this moisture content expands, causing the concrete to crack or spall. Spalling refers to a pocket of concrete that has crumbled into fine particles through the exposure to heat. Spalling can reduce the critical mass of the concrete-the mass used Fig. 2-13. Concrete masonry units are a popular building for strength. Steel rebar that becomes exposed to a material for walls. fire after spalling can easily conduct heat within the 24 Chapter 2 ESSENTIAL BUILDING CONCEPTS Individually, brick, CMU, and stone have excel­ Reheating a thermosetting plastic will change the lent fire-resistive qualities. Oftentimes masonry composition and will likely result in breakdown. walls will still be standing after a fire has ravaged Most plastics are considered ductile. the interior of the building. Even though the wall is still standing, the loss of the roof means the wall The building industry is the second leading no longer has the compressive forces needed for consumer of plastics in the U.S. ( packing and strength-the wall is unstable and can collapse shipping is first). Since the mid-1960s, plastics have quickly when an eccentric load (like wind) is been increasingly used for just about everything applied. The masonry wall also has an Achilles' except the structure itself. That is now changing. heel: the mortar used to bond the individual units. Plastics are now being used to reinforce wood and Mortar is subject to spalling, age deterioration, and concrete. Several all-plastic buildings have been washout. During a fire, masonry blocks (or bricks) constructed to demonstrate the potential of plastics can absorb more heat than the mortar used to bond to replace wood, steel, and concrete. Clearly, the them, creating different heat stresses that can crack trend is for more plastics to be included as building the binding mortar. Whether from age, water, or materials. From a firefighter's perspective, this trend fire, the loss of bond causes a masonry wall to can have negative impacts on building stability become very unstable. during fires as most plastics melt at relatively low temperatures and emit very explosive gases that add tremendous heat-release rates when the hydro­ Composites carbons burn. New material technologies are posing inter­ Carbon-fiber reinforced polymer (CFRP). CFRPs esting challenges for the firefighting community. are composite materials that include a reinforcing The term composite can be used for many things material (the carbon fibers) that is bound together but in this case refers to a combination of the with a polymer (like epoxy). Carbon fiber is amaz­ previously mentioned basic materials, as well as ingly strong and can be woven or shaped in many various plastics, glues, exotic metals, and assembly forms. To put things in perspective, the carbon methods. One thing is certain, these materials content of steel helps gives the steel its strength. are designed to offer maximum strength with Manufacturers have figured out a way to take that minimal material mass-a dangerous proposal essential strength element, crystallize the bonds, in the structural firefighting environment. Given and form a fiber. The fiber is a fraction of the that, several composites are commonly used for thickness of human hair, pliable, and heat, corro­ building materials. sion, and rot resistant. Because of cost, CFRPs are not prevalent as a building construction mate­ Plastic. Simply stated, a plastic is synthetic or rial, although engineers are finding application of semisynthetic material that is made of moldable CFRPs for reinforcing concrete and steel. As the polymers (a molecule with many connected atoms). cost comes down, more applications will be found. Most plastics are derived from petroleum. The hydrocarbon chemical chains found in crude oil Under fire conditions, CFRPs offer initial heat can be altered with other chemicals to form many resistance until the polymer degrades. Once the different products. While there are thousands of actual carbon fibers are exposed to flame, they chemically-named plastics used in everyday life, separate and release microscopic carbon particles most plastics can be divided into thermoplastics that can burn. The particulates from CFRP smoke and thermosetting plastics. Thermoplastics can be are especially destructive to microelectronic circuit heated and reshaped without losing the inherent boards as they can form a conductive path between composition found in the plastic. Thermosetting components. plastics use heat to harden or set the plastic. 25  f The Alt of Reading Buildings The expanding use of composites for many To finish this section, it is important to know I types of building materials will certainly change that technological advances in material science I the basic perceptions of fireground operations that are ongoing and new combinations of materials have been taken for granted for many years. This are finding their way into buildings. In chapter 6, I is a primary reason why firefighters must keep we further explore some of the evolving building abreast with technology and resultant changes in methods and the material composites that are being building construction, particularly in your area used as well as some firefighter tactical consider­ of responsibility. ations we face in them. Quick summary CHAPTER REVIEW EXERCISE Concrete has been a primary building material for hundreds of years and is a combination of, sand and aggregate, water, and Portland cement. Answer the following: Concrete is known for its high compressive strength (hence its use in foundations), but has 1. What is a "load"? poor tensile and shear strength. 2. How are loads imposed on materials? Steel is added to concrete as reinforcement 3. What are the differences between compression, for applications that require tensile and shear tension, and shear forces? qualities. Reinforced concrete can be formed as monolithic, pre- or post-tensioned, or precast. 4. What influences the suitability of a material for a given building application? Concrete will absorb and also radiate stored heat, and can spall when exposed to heat from fire. 5. How are brittle and ductile materials different? Masonry often refers to brick, tile, concrete 6. Describe the relationship of surface-to-mass block, and stone. ratio and fire degradation on building materials. Similar to concrete, masonry products have 7. Match the wood products in the left column to good compressive strength, and are generally the associated wood type in the right column: resistant to heat from fire. Plywood Engineered In particular, CMUs (also known as cinder wood product blocks) have become very popular due to their OSB strength, resistance to fire, and minimal ongoing LVL Traditional maintenance. These factors are a benefit to the Sawn Douglas fir wood product fire service. Glulam Standard materials that are commonly used in Heart wood Native lumber building construction are constantly evolving in concert with advances in technology, and 8. At what temperature does hot-rolled (extruded) as a result, the term composites is becoming steel begin to fail? more familiar to building construction method­ ologies and materials. Two notable examples 9. What is meant by "spalling" and how is it are plastics (being added to engineered wood caused? products) and carbon-fiber materials. 10. During a fire, two conditions will cause a masonry wall to become unstable. List the two conditions. 26 ANATOMY OF A BUILDING-A MAP OBJECTIVES Define foundations, columns, beams, and connections. Identify the parts of a truss. List the three types of structural connections. Define structural assembly. Define the difference between panel, partition, and curtain walls. Describe the structural hierarchy. COMMUNICATION SKILL-BUILDING FOR BUILDINGS hen launching an aggressive interior fire attack on a building, you and your crew are relying on building integrity-that is, the building's resis­ tance and reaction to heat, fire, smoke, firefighting impacts, and gravity. We know that a building can only stand so much fire and heat assault before gravity takes over and the building starts to fail. Before you can predict building failure, it is important to understand basic building anatomy and the appropriate terms and concepts that can help you communicate collapse concerns. In the previous chapter we made the case for understanding common building construc­ tion/engineering terms and materials. In this chapter, we strive to help you understand the components of a typical building. W hile this sounds easy enough, the importance of this understanding and the use of appropriate terms and labels cannot be over emphasized. For example, confusing the term column with beam, or truss with rafter, can have detrimental consequences when communicating with an incident commander or safety officer. Likewise, understanding the inter­ play of structural components can help you make better decisions regarding safe or unsafe positions from which to operate or complete your tactical assignment. 29  The Art of Reading Buildings Lastly, the critical communication information At first glance, you may argue that walls, floors, found in this chapter can help you understand the and roofs are missing from the structural elements language used in the following chapters of this list. In some cases, walls, floors, and roofs are book where we talk about different construction structural elements, but not always. In many build­ types and the considerations related to fighting fire ings, you can remove the walls, floors, and roofs in, on, or around buildings. and the building will still stand. This is where the phrase "structural elements" differs from building coverings, finishes, and features. If you remove or damage a structural element, the building will STRUCTURAL ELEMENTS begin to fail. Granted, this may be semantics, but once again, the use of appropriate language Buildings contain many elements, components, will help you better communicate issues during features, finishes, and systems that come together your firefight. to form an enclosure for a given purpose. Many of Understanding whether or not a building com­ the aforementioned things are not necessary for the ponent is a structural element begins with the building to stand with integrity. From a firefighting definition of foundations, columns, beams, and perspective, attention must be given to the under­ connections and the interplay each must have. In pinnings of a building that must be monitored when this chapter we define structural elements (and how evaluating the building's integrity to stand-the they affect each other), then define other building structural elements. Further defined, structural components. In later sections of this book, we dis­ elements are those essential underpinnings of cuss the firefighting concerns associated with them. a building that allow it to stand erect and resist imposed loads and gravity (fig. 3-1). Structural elements work together to deliver all loads to Foundations earth. In the simplest form, there are four structural elements of any building: The foundation of a building can be defined as the building's anchor to earth and base for all Foundations elements built above that anchor. This anchor Columns must have properties that allow it to deliver all imposed building loads and deliver them to earth in Beams compression. Further, foundations must be designed Connections in a way that will help keep a building level, and resist the chance of sinking, twisting, or leaning. The foundation of a building can be formed using footers, foundation walls, slabs, and/or pilings. Footers: Footers (or footings) are weight-distrib­ uting pads that serve as the bottom of foundations. Footers are typically the lowest/deepest part of any building and directly contact earth. Footers can serve as a perimeter base for slabs and foundation walls or as solo pads to support columns (fig. 3-2). Foundation walls: These are walls installed below Fig. 3-1. Structural elements work together to deliver grade to serve as structural support for other struc­ a building's loads to the earth. tural elements and also to hold back soil and other materials. Foundation walls also typically serve 30 Chapter 3 ANATOMY OF A BUILDING-A MAP as the perimeter basement walls of a building. Most foundation walls are poured-in-place, steel rebar reinforced concrete, although masonry block, precast panels, or heavy timbers and planking can be used. Often, foundation walls incorpo­ rate footers. Fig. 3-3. A column delivers the weight of beams and other columns to a foundation. The load that a column can carry is dependent on many factors such as the material used, the Fig. 3-2. Footers, solo pads, and foundation walls length (height if vertical), and its cross-sectional shape. The cross-sectional shape of a column is important because columns carry compressive Slabs: Slabs (when used as a foundation) are flat loads axially through their length. The best shape horizontal elements that simply rest on the ground. for a column is one where the compressive load Some call this a slab-on-grade foundation. Most is shared equally (and spread further) from the slab foundations still incorporate footers. center axis of the material being used. Rectangles, squares, and cylinders (like a pipe) are preferred Note: Foundation walls and slabs are covered from ways of shaping columns (fig. 3-4). another viewpoint in chapter 7. Pilings: Pilings are vertical posts that are driven AXIS AXIS AXIS down into the earth to serve as the foundation or foundation anchor of buildings. Columns A column is defined as any structural element that is loaded axially, along its length, in compres­ sion. In most buildings, columns deliver the weight Fig. 3-4. Rectangles, squares, and cylinders are preferred of beams and other columns to the foundation. ways of shaping columns. Columns can take on the form of a wall or a post - ---- ·-----·---------·----- - - (fig. 3-3). When a wall is used as a column, it is often referred to as a load-bearing wall or, more Hot-rolled steel is often used for hollow columns appropriately, a wall column. In earlier times, the that are either square or cylindrical, or a cross­ term pillar was used synonymously with column. sectional shape like the letter "H" (hence the term More recently, a pillar is defined as a freestanding H-column). The use of an "I" cross-sectional shape vertical post, monument, or architectural feature. is not ideal for a column-it becomes prone to buckling because the load is not shared equally 31  The Art of Reading Buildings ,- around the axial center. Hollow columns incorpo­ through its length. Figure 3-6 shows horizontal rate a cap of some type to help connect the column columns that are being used to keep two brick wall to beams and distribute the load evenly around columns from falling into the alleyway. Horizontal the perimeter shape. Concrete masonry blocks (or columns are called struts and diagonal columns CMUs) used to build a wall column are typically are called rakers. A typical application for a raker rectangular, suggesting that they don't disperse is a post driven diagonally into the ground to help weight equally around their center axis. Closer hold shoring for earthen excavations and trenches. examination of these CMUs shows that they are Exterior load-bearing walls under construction are formed with multiple squares (two, three, or four often wind-braced with rakers until a floor or roof cubes) that are joined together to achieve uniform load is applied. distribution. Columns are considered a critical element of any building-they hold up floors and roofs. For that reason, columns must be designed and built with resistance to lateral forces that could knock them over. The material used and the application of compressive load through the length of the column help to resist lateral loads. Buttress and pilaster: A buttress is an exterior wall bracing feature used to assist with lateral forces created where roof beams or trusses rest on a / PILASTERS ----- - BUTTRESSES wall. Buttress are structural in nature and can take on numerous shapes (a diagonally ascending Fig. 3-5. Buttresses are used to counteract lateral forces in a stack of stone or brick is most common, as in vertical wall and pilasters are used to strengthen a wall. fig. 3-5). Some textbooks use the term pilaster to also describe a buttress. Historically, a pilaster is a decorative column that protrudes in relief from a wall to give the appearance of a separate post column. Over time, the fire service began using the term pilaster to describe any interior or exterior thickening of wall used to add lateral support for roof beams and trusses. For our purposes, we accept that both pilasters and buttresses can be structural and that they can be differentiated by shape. A pilaster appears as an interior or exterior vertical stack that thickens a wall column, whereas a buttress is a separate, diagonally-stacked brick, stone, or concrete wall that protrudes perpendicularly from the wall column supporting the roof. While most columns in a building are vertical in attitude, it's important to note that columns can also be diagonal or horizontal. The guiding Fig. 3-6. Horizontal columns that are compressively loaded principle is that a column is compressively loaded through their length 32 Chapter 3 ANATOMY OF A BUILDING-A MAP can carry for a given span is actually proportional Quick summary to the square of its depth. If you double the top-to­ bottom depth of the beam, you can carry four times Buildings are comprised of many structural the load. If you triple the depth, you can carry nine components that combine to form an enclosure times the load. For example, a simple 2 x 4 in. wood for a given purpose. beam might carry 50 lb. If you attach a second The primary structural elements of a building 2 x 4 next to it-you merely double the weight it are foundations, columns, beams, and connec­ can carry to 100 lb. If you replace the side-by-side tions. These elements work together to transfer attached beams with one that is 2 x 8 in. (increase all loads to the earth. the depth), it can carry 200 lb (fig. 3-8)! The length that a beam can span is directly proportional to its A foundation is a building's anchor to the earth. depth. If you double the top-to-bottom beam depth, Columns are a primary structural element that you can double the span. These depth relationships delivers the weight of beams and columns to are simplistically sound although the engineering a foundation. community uses complex formulas that take into Buttresses and pilasters are used to enhance account shapes, materials, spacing, and safety the stability of an exterior wall. Buttresses factors when designing beams. assist with lateral forces and pilasters help to strengthen a wall. NEUTRAL PLANE Beams Beams are used to create a covered space­ z z usually between columns. Roofs, floors, and most z 0 0 loads placed in buildings are picked up by beams ::e:) iii w SHEAR SHEAR "'w iii a: a: 8...I a. and delivered to columns that deliver a load to the ::e0 foundation. By definition, beams are structural 8 elements that deliver loads perpendicularly to their Fig. 3-7. Beams transfer load using opposing compressive imposed load and in doing so, create opposing and tensional forces. forces within the element. Any load placed on a beam (and the weight of the beam itself) causes the beam to deflect. That is, the top of the beam is 2wx4dx20L 4wx4dx20L 2wx8dx20L subjected to a compressive force while the bottom LOAD of the beam is subjected to tension (fig. 3 7).- In between the compressive and tensile load is a t LOAD neutral plane. The neutral plane creates an area where there are no stresses. This is the area where t a small hole can be punched through the beam to accommodate piping, electrical wires, or for other purposes. A B C SINGLE BEAM DOUBLE BEAM WIDTH DOUBLE BEAM DEPTH The distance (or depth) between the top of the = DOUBLE STRENGTH = 4X STRENGTH beam and the bottom of the beam dictates the Fig. 3-8. Beam strength is proportional to its depth. amount of load the beam can carry or the distance the beam can span. The amount of load that a beam 33  The Art of Reading Buildings Beams are typically labeled by their applica­ Rafter: A sloped wood joist that supports roofing tion and by their designed shape/material arrange­ coverings between a ridge beam and wall plate on ment. We'll discuss shape/material a bit later in peaked and hipped roofs. this chapter. Beams labeled by application include the following: Ridge beam: The uppermost beam.of a pitched roof. Rafters attach to the ridge beam. Simple beam: A beam supported by columns at the two points near its ends. Purlin: A beam placed horizontally and perpen­ dicularly to trusses or beams to help support roof Continuous beam: A beam supported by three or sheathing or to hang ceilings. more columns. Suspended beam: A beam that has one or both Cantilever beam: A beam supported at only one ends supported from above by a cable or rod end. (Or a beam that extends well past a support in (sometimes called a hung beam). such a way that the unsupported overhang places the top of the beam in tension and the bottom Most beam applications are such that the in compression.) beam is laid in a horizontal attitude. Beams can also be vertical. A retaining wall for landscaping Lintel: A beam that spans an opening in a load­ or grading is essentially a vertical cantilevered bearing wall, such as over a garage door opening beam. The same can be said for communication (often called a "header"). Lintels can also be antennas. A highway billboard has to be built like commonly found over windows and doors in a beam to withstand wind-another example of a unreinforced masonry construction and in newer vertical beam. CMU construction (fig. 3-9). Beams are further classified by their material arrangement and shape. A solid wood or reinforced concrete beam is simply that-a solid beam. A steel beam that is cross-sectionally shaped as a rectangle is also considered a solid beam, although techni­ cally it is hollow inside. Beams can also be created or built with various shapes and pieces. The term "I-beam" reflects the shape of the beam viewed from either end. The top of the "I" is known as the top chord (or flange); the bottom of the "I" is the bottom chord (or flange). The piece used to span the Fig. 3-9. Lintels are used to span an opening in a load­ distance between the chords is known as the web bearing wall. (sometimes called the stem). The two most popular I-beams are the popular engineered wooden Girder: A beam that carries other beams. I-beams (see fig. 8-12) and extruded steel I-beams. Both of these beams have a solid or closed web, Ledger: A beam attached to a wall column that although small holes can be punched through the serves as a shelf (ledge) for other beams or neutral plane of the web to accommodate utilities. building features. Truss: A truss is an engineered structural element Joist: A wood or steel beam used to create a floor that uses groups of rigid triangles to distribute and or roof assembly that supports sheathing or decking. transfer loads. The triangles create an open web Joists span between primary supporting members space. Trusses are used in lieu of solid beams in such as foundations, load-bearing walls, or struc­ many buildings. Although the initial perception of tural beams. this definition may seem slightly complex, the basic 34 Chapter 3 ANATOMY OF A BUILDING-A MAP design of a truss is rather simplistic and has been member is called a king post truss, which consists used in this country since the early 1800s. The basic of two angled supports that intersect a common concept of the truss is not new, but the materials vertical support (fig. 3-IOB) that is joined to the that are currently being used in its construction bottom chord. are new as heavy timbers have been replaced with lightweight wood and/or metal structural members. This has resulted in a significant change within the building industry and has dramatically changed how many buildings burn and fail when exposed to heat and/or fire. Because truss construction has, in many instances, become the norm for many struc­ tural applications, let's take a quick look at the concept of a truss. BOTTOM CHORD A truss is nothing more than a structure that Fig. 3-10A. A single triangle is an example of a simple consists of one or more triangles formed by straight planar truss. ----------- members whose ends are connected at j