Fundamentals of Residential Construction (3rd Edition) PDF

Fundamentals of Residential Construction  FUNDAMENTALS OF RESIDENTIAL CONSTRUCTION THIRD EDITION Ed w ard A ll e n a nd Rob Thal lo n Featuring the Drawings of Joseph Iano John Wiley & Sons, Inc. This book is pr...

Fundamentals of Residential Construction  FUNDAMENTALS OF RESIDENTIAL CONSTRUCTION THIRD EDITION Ed w ard A ll e n a nd Rob Thal lo n Featuring the Drawings of Joseph Iano John Wiley & Sons, Inc. This book is printed on acid-free paper. o Copyright © 2011 by John Wiley & Sons. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8600, or on the web at www.copyright.com. 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TH4811.A463 2011 690’.837–dc22 2010050395 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1  PART FOUR Alternative Construction Systems  22 M ult ifamily Cons tr uc t ion History Systems Special Issues Accessibility Foundations Site Management Framing Green Issues for Multifamily Housing Acoustical Separation Fire Protection Regulating Combustibility Fire Sprinkler Systems Fire Separation Fire Blocking 517 518 / Part Four Alternative Construction Systems History It has only been in the last few hun- dred years that a significant percent- age of housing has been built in the form of multifamily structures. Prior to the Industrial Revolution, people came together in villages and towns primarily for collective agricultural convenience, defensive purposes, and the simple desire for human in- teraction. Most group housing in an- cient times consisted of collections of attached buildings that housed extended families. These were often fortress-like with defensible access and minimal windows on the exteri- Figure 22.1 or. Examples can be found in Africa Taos Pueblo was certainly one of the earliest multifamily buildings on the continent. (the Gurunsi of Upper Volta), Asia Started around AD 1000, it grew to approximately its present size by the year 1400. (the silo-cave courtyard housing of With over 600 rooms and up to five stories tall in places, the large adobe complex northern China), and North America served for centuries as a trading and religious center and is the longest continuously (the pueblo dwellings of the Ameri- occupied building on the continent. (Photo by Rob Thallon) can Southwest). By the Middle Ages, great fortified cities could be found across the globe, including London, Rome, Dubrovnik, and Kyoto. cities soon followed this example, acts included provisions for flush However, true cities (where the and by the turn of the next century, toilets and effective garbage remov- need for density is greatest) housed townhouses were prevalent in most al. In 1927, the Uniform Building a relatively minor proportion of cities across the country. In the East Code was enacted, becoming the the human population until quite and Midwest, most were constructed first of several major codes govern- recently. In 1700, only about 3 per- of brick or stone, while wood was the ing the design and construction of cent of the world’s population lived material of choice in the West. all buildings, including residential, in settlements of more than 5000 The townhouse type, originally de- for the remainder of the 20th cen- people. The biggest shift of popula- veloped for prosperous families with tury. During this period, separate tions toward cities was the result of country property who also wanted a one- and two-family dwelling codes the Industrial Revolution, which residence in town, was later adapted were developed, but the regulation started in the mid-18th century. The for middle-class urban residents and of multifamily buildings remained opportunities to find work in facto- for worker housing in mill towns under the jurisdiction of the parent ries brought swarms of people to the across the continent. To conserve codes. In 2000, the various regional cities. By 1900, the percentage of land, the connected houses were code organizations were combined people living in cities had climbed made smaller and narrower, leading into the International Code Coun- to more than 13 percent worldwide. to the so-called tenement houses with cil with the intention of providing a This influx of people to urban areas central rooms that lacked ventilation single set of codes for all of North created a boom in worker housing and daylight (Figure 22.2). America. The first code issued by this and, with it, overcrowding and prob- In New York, a series of Tene- new organization was the 2000 In- lems with sanitation, ventilation, fire ment House Acts starting in 1867 ternational Residential Code (IRC), safety, and privacy. were designed to alleviate the squa- which regulates one- and two-family In North America, Philadelphia lor associated with worker housing. residences and townhouses up to was one of the first cities to adopt the These acts required multifamily three stories in height. The 2003 European pattern of urban living. In housing to have shared privies in International Building Code (IBC) that city, as early as 1700, deep, nar- the building, windows opening to pertains to all other buildings, in- row lots promoted attached housing fresh air for every room, and escape cluding multifamily buildings that called townhouses. Other East Coast routes in case of fire. By 1901, the are not townhouses.  Chapter 22 Multifamily Construction / 519 Figure 22.2 It is interesting to note that the The social codes differentiate residential con- issues related to struction by type. One- and two-family overcrowding in residences and townhouses are gov- tenement housing erned by the residential code (IRC), were discussed in this and other types by the more general edition of Harper’s building code (IBC)—presumably Weekly from the because issues of emergency egress summer of 1883 and and access are quite different in the in many other media two groups. Townhouses (also called of the time. row houses) are multifamily buildings in which each unit has a private en- trance from a ground-level public out- door space. In this sense, a townhouse building is essentially an expanded duplex (Figure 22.3). Units in other types of multifamily construction, on the other hand, do not necessarily connect with the ground. Flats, for example, are organized horizontally so that some units do not touch the ground, and this is the reason that emergency access is more compli- cated. Thus, this type of multifamily building is grouped in the building codes with other large multiuse and tall buildings in which issues of public health and safety are greater because of the greater height, area, and over- all scale (Figures 1.24 and 22.4). Figure 22.3 A three-story townhouse-style student housing project. Apartments are arranged around a central courtyard with semiprivate terraces, and ground-level parking is out of sight on the perimeter. Each apartment has a ground-level entry, and there are no elevators. Build- ing forms and the use of color create visual interest. (Photo by Rob Thallon) 520 / Part Four Alternative Construction Systems Figure 22.4 Four stories of wood-framed construction over a single-story concrete plinth in San Francisco. Apartments are arranged as flats, one stacked over another with horizontal circulation that is reached by an elevator. (Photo by Rob Thallon) Figure 22.5 U.S. housing production measured in thousands of units. Multifamily housing accounted for more than 18 percent of residential construction, which amounted to more than $50 billion worth of construction for the year documented. (Source: U.S. Census Bureau, Housing Completions, 2000) Codes regulate the areas and height limit is related to accessibil- off a corridor (hotels, dorms, apart- heights of multifamily buildings. ity by a fire rescue hook-and-ladder ments). The area restrictions are primarily truck parked at the lowest level of As the population continues designed to limit the spread of fire, the building. to migrate to urban settings, the with smaller buildings having lower Multifamily buildings are also importance of multifamily hous- fire-resistance requirements. For defined by ownership. Hotels and ing will increase. In the year 2000, this reason, buildings that appear to apartment buildings are under sin- the worldwide population living in be very large are often divided into gle ownership, and the individual cities had increased to 50 percent. more than one building with fire-rat- units are rented or leased. Condomin- Currently in North America, the de- ed walls that extend from foundation iums are a variation on this model in sign and construction of multifamily through roof. Until quite recently, which the units are owned individu- housing employs about 20 percent most wood-framed residential build- ally, but the building and land are of professionals in the related fields ings were limited to three stories owned jointly, and the maintenance (Figure 22.5). Thus, it is important to in height, mostly for reasons of fire of the common spaces and exterior understand the distinctions between safety. But as fire safety improved of the building are managed by an this type of housing and the more and the economic pressure to add owners’ association to which each common single-family house. another rentable story combined owner pays dues. Although some multifamily hous- with the environmental argument Circulation systems also differ- ing occurs in high-rise buildings, most for greater efficiency, codes were re- entiate multifamily buildings. Build- is found in low- or midrise buildings laxed. Now four and even five stories ings can have private entrances from of five stories or less. The high-rise of wood frame construction are al- public outdoor spaces (townhouses), component is not covered here be- lowed. Many jurisdictions allow this shared vertical circulation with mul- cause the construction materials and wood frame construction to be situ- tiple stacked units off a single stair methods are more related to heavier ated above a single-story fire-resistant (walk-ups, flats), or shared horizon- types of construction and are thus be- concrete structure. The ultimate tal circulation with multiple units yond the scope of this book.  Chapter 22 Multifamily Construction / 521 Figure 22.6 A four-story wood-framed apartment building built in 2009. All four levels are entirely residential, and parking is adjacent on a surface lot. The foundation for this building is a slab on grade. (Photo by Rob Thallon) (a) (b) cause multifamily buildings are gen- Special Issues erally taller and larger than single- The fact that multifamily units are family residences, the structural loads tightly packed, often one above an- are greater, and this leads to the need other, leads to a number of problems for stronger foundations. Footings that must be solved with special de- are wider, stem walls are thicker, and sign and construction. Fire safety re- more reinforcing is required. quires particular attention. Natural Often, the foundation type is ventilation and daylight are challeng- dictated by the need for parking. ing. Acoustical privacy is difficult be- Where land is available for sur- (c) cause of the proximity of units. Struc- face parking, the building founda- tural issues become more extreme as tion is typically a concrete slab on the height of the building increases. grade—by far the least expensive Because these issues are complicated foundation option (Figure 22.6). A and a great number of lives are af- slab-on-grade foundation for a large fected by the response to them, most multifamily building is essentially a jurisdictions require that a licensed larger, stronger version of this foun- architect oversee the design of mul- dation type that would be built for tifamily residences governed by the a single-family residence. The ad- International Building Code. vantages of simplicity and lower cost as compared to other foundation types apply to multifamily buildings Figure 22.7 Foundations the same as they do for single-family Three ways to configure structural con- residences. However, greater gravity crete parking and/or commercial space Foundations for multifamily build- loads and lateral loads on the larger below wood light frame housing: (a) Park- ings must meet the same require- building require the footings to be ing below grade with four or five floors of ments as those for single-family wider and thicker. The increased housing above. (b) Commercial space and/ residences—supporting the loads likelihood of differential settling or parking at grade with four or five floors of the building while separating the because of a larger footprint will of housing above. (c) Parking below grade structure and the occupied spaces require a stronger foundation for with commercial and/or parking at grade from the moisture of the ground. Be- uneven or weak soils (Figure 22.7). and four or five floors of housing above. 522 / Part Four Alternative Construction Systems Figure 22.8 A slab-on-grade foundation being prepared at one end of a large multifamily building. All of the plumbing drains and the electri- cal conduits have been installed and covered with gravel. Workers are compacting gravel, installing reinforcing wire, and pouring the slab in sections. (Photo by Rob Thallon) Figure 22.9 Concrete footings with rebar projecting for concrete foundation walls for an underground parking garage. The sides of the excavation have been draped with plastic to control erosion. Note the concentrated rebar at intervals along the perimeter walls to strengthen the wall for columns that will support upper floors. (Courtesy Rowell/Brokaw Architects) In urban contexts where land the foundation for the residential usually reduced in length in order to is most expensive and development construction. keep the vehicle level fairly open for must be dense, the favored option for When vehicle parking is located ventilation (Figure 1.24). The scale required parking is usually to locate it beneath the residential units, a foot- and relative openness of a parking below the building. Where mixed-use ing and stem wall foundation are gen- level makes design for lateral stabil- zoning allows commercial activities erally employed (Figure 22.9). If the ity more difficult, and the strategic in the same building with residential parking is below grade, foundation location of sufficient lengths of shear uses, the commercial uses can be lo- walls are long and continuous and wall within the structure is important cated on the ground floor with the (along with backfill) will effectively (Figures 22.10 and 22.11). residential stacked above. The struc- stabilize the building. With parking at When residential units of ture of the commercial story provides grade, however, foundation walls are wood light frame construction are  Chapter 22 Multifamily Construction / 523 constructed above a parking garage, the deck between the two must have sufficient resistance to the passage of fire that the apartments are unlikely to burn in the event of a prolonged petroleum fire at the parking level. The code typically specifies that this deck must have a fire-resistance rat- ing of 2 hours. The deck is also called upon in most instances to accept the loads from the walls and columns in the units above and to transfer them to the columns and walls in the park- ing garage. It is sometimes possible to plan the building so that the load- bearing walls and columns of the units are aligned over beams in the deck below, but this can be a difficult task, given that the column locations in the garage are largely dictated by automobile dimensions and not by residential layouts. Figure 22.10 The fire-rated deck may be con- The underground parking garage shown in Figure 22.9 with concrete foundation walls structed in several ways. One is to use completed and formwork for a posttensioned slab started at the edges. The rebar heavy timber beams and wood joists projecting from the top of the walls will be bent to tie in with rebar in the structural and decking (Figure 22.12). Another slab. Note the concrete shear walls perpendicular to the perimeter walls. (Courtesy is to use structural steel framing and cor- Rowell/Brokaw Architects) rugated steel decking (Figures 22.13 and 22.14). A concrete slab is poured over the steel decking to create a smooth floor above (Figure 22.15). In a steel frame, and in some instances in a heavy timber frame, fireproof- ing material must be applied to the members to achieve the required fire-resistance rating. Heavy timber or steel frame construction with decking that spans between framing members works better on a tight site than site-cast structural slabs because work at the parking level can proceed without being obstructed by a forest of shoring columns. Figure 22.11 Workers forming perpendicular reinforced concrete shear walls at the ground level of a future five-story building. A posttensioned slab will be formed and poured at the top of the walls, and four stories of wood frame construction will be built on top of the slab. The shear wall to the left has rebar, conduit, and sleeves in place and is awaiting the final formwork. (Photo by Dee Etzwiler) 524 / Part Four Alternative Construction Systems Figure 22.13 Steel beams form the structure over this below-grade parking garage. The beams are supported at the perimeter on reinforced concrete masonry unit (CMU) pilasters integrated with the CMU retaining wall. The selection of steel framing allowed the parking garage to be used almost immediately as work space. If a postten- sioned slab, with its virtual forest of shoring, had been selected, the space would not have been available for at least several weeks. (Photo by Paul Dustrud) Figure 22.12 A hybrid structure supports the housing above this below-grade parking garage. The columns are concrete, long beams are steel wrapped with fire-rated insulation, and shorter beams are glulam timbers. Sprinklers protect the structure. Note that plumbing drains must pass under the beams. (Photo by Rob Thallon) Figure 22.14 A steel girder is supported by a concrete column adjacent to a CMU shear wall in this below-grade parking garage. The tops of the beam and shear wall are flush and support a steel pan deck that, in turn, will provide support and formwork for a concrete deck above. The deformations in the corrugated steel pan deck allow the deck to physically bond with the concrete slab so that the steel and concrete work in concert to resist deformation of the assembly. The depth of the steel beam will reduce clearance in comparison with a posttensioned slab such as the one shown in Figure 22.37. All of the steel will need to be coated to protect it from fire. (Photo by Paul Dustrud)  Chapter 22 Multifamily Construction / 525 Figure 22.15 Concrete workers place the slab on the steel deck shown in Figure 22.14. Notice the simplicity of the reinforcing and other preparations for concrete in comparison with the posttensioned deck shown in Figure 22.21. Penetrations of this deck for plumbing and other utilities will be made later with a concrete drill. (Photo by Paul Dustrud) Concrete deck systems are effec- tive and often economical for this application. Precast concrete ele- ments, usually hollow core planks, are produced in a factory and hauled to the site, where a crane lifts and places them side by side (Figure 22.16). A concrete topping that is about 2 inches (50 mm) thick is usually poured over the slabs to pro- duce smooth floors and increase the fire-resistance rating (Figure 22.17). Precast concrete components are often insufficient in themselves to achieve the required fire-resistance rating. An applied fireproofing prod- uct, usually based on noncombus- tible fibers with a noncombustible binder, can be applied to the under- side of the components to increase the rating. Site-cast concrete decks are also widely used in this situation. Their greater thickness of concrete within which the reinforcing steel is em- bedded compared to that of precast Figure 22.16 elements is generally sufficient to A four-story apartment building under construction in a dense urban area. The ground floor achieve the required fire resistance is constructed of masonry walls with precast concrete columns, beams, and hollow core without an applied fireproofing coat- planks. (Note the end of the planks just above the bed of the truck.) The upper floors are ing. Among the various site-cast fram- constructed of premanufactured wood panels. The entire structure except for the masonry ing systems, the one most commonly foundation is manufactured off-site and lifted into place with a crane. (Photo by Rob Thallon) chosen for this application is the 526 / Part Four Alternative Construction Systems Precast hollow Figure 22.17 core slabs, grout, Precast hollow core concrete deck system and topping with a site-cast topping. Grout in the upper half of the hole anchors the beam to the reinforcing bar that projects from the column Reinforcing loops prevent vertical bar from tearing out of beam Mastic in the Precast column lower half of the hole allows for structural movement posttensioned two-way flat plate (Figure the necessary resistance to bend- clearance for parking or commer- 22.18). This is a concrete slab of uni- ing. A concrete thickness of 9 to 10 cial activities. The disadvantages of form thickness that is reinforced in inches is generally sufficient to span this and other site-cast systems is that both the longitudinal and the cross- columns spaced 29 or 30 feet apart they require construction of form- wise directions with a grid of steel in both directions, which are typical work on the site—consuming valu- tendons. A tendon is a cable made spans for a parking garage. Postten- able on-site schedule for the erection of very high strength steel wires. It is sioned slabs (PT slabs) are sophisti- of the formwork and usurping valu- enclosed in a plastic tube that is filled cated structural elements that must able work space as it supports the with a lubricant to protect the cable be carefully engineered and con- curing slab for several weeks after it from corrosion and to assure that structed in order to perform as in- has been poured (Figures 22.22 and it can move freely within the tube. tended (Figure 22.21). 22.23). The tendons are placed in the form- A posttensioned concrete slab Residential units are often built work, and the concrete is poured and supplies an ideal foundation for above commercial establishments. cured around them. When the con- multiple floors of lightweight wood The most common structural solu- crete has reached sufficient strength, construction above because it can tion to this situation is to construct workers with hydraulic jacks stretch be made strong enough to support the ground-floor commercial space each cable to a very high force, and the gravity loads of the building no with concrete columns and concrete the ends of the stretched cable are matter where they fall. Another sig- or masonry shear walls that support anchored to the edges of the slab nificant advantage of the PT slab is a posttensioned concrete slab ceil- (Figures 22.19 and 22.20). The force that it is relatively thin as compared ing structure above. The slab serves of the many tendons compresses to other systems with similar span- as the floor of the first level of resi- the concrete in both directions of ning capacity because it has no dential spaces above. This entire first- the building, which gives the slab beams on the underside to impede floor concrete structure is referred  Chapter 22 Multifamily Construction / 527 Figure 22.18 A plan and two larger-scale sections of the tendon layout in a two-way flat plate floor with banded posttensioning. The number of tendons running in each of the two directions is identical, but those in one direction are concentrated into bands that run over the tops of the columns. The draping of the tendons is evident in the two section drawings. Building codes require that at least two distributed tendons run directly over each column to help reinforce against shear failure of the slab in this region. In addition to the tendons, conventional steel reinforcing is used around the columns and in midspan, but this has been omitted from these drawings for the sake of clarity. 528 / Part Four Alternative Construction Systems Figure 22.19 Forming and placing rebar for a structural post- tensioned slab at the second-floor level. The form deck extends beyond the edges of the slab in order to provide work space at the slab edge. Note the end anchorage fittings for posttensioned tendons that are screwed to the formwork at the lower right of the photo. The metal brackets attached to the formwork just above will connect a steel balcony railing after the slab is finished. (Courtesy of Rowell/ Brokaw Architects) Figure 22.20 Workers are tightening the tendons of this structural posttensioned slab just 5 days after the slab was poured. The concrete was required to have reached a compressive strength of 3000 psi. The tendons, which are about 100 feet long in this case, were stretched about 10 inches to achieve the required stress. The stretch length of the tendons is calculated by engineers so that workers can gauge the stress by merely measuring the movement of a mark on the cable. (Courtesy of Lease/Crutcher/Lewis Builds)  Chapter 22 Multifamily Construction / 529 Figure 22.21 Rebar, tendon cables, and utility sleeves are being installed in preparation for pouring concrete for a posttensioned slab. It is extremely important to accurately locate vertical penetrations of a PT slab for pipes and wires before it is poured. The concrete cannot be drilled or sawn for an opening without first x-raying the slab to be sure that PT tendons are not damaged. Note the complexity of preparations in relation to the metal pan deck system shown in Figure 22.15. (Courtesy of Pivot Architecture) Figure 22.22 Stacks of forms await placement above, where they will support the casting of a posttensioned slab. The structural frame of the modular forms and the stringers that support them are made of aluminum to minimize their weight for ease of handling. The vertical shoring members are steel and have a drop-head feature that allows them to remain in place to support the curing slab while the rest of the formwork is removed. (Photo by Dee Etzwiler) 530 / Part Four Alternative Construction Systems Figure 22.23 The forms for a two-way flat plate structural slab are set lower around columns in order to form a thicker slab called a drop panel around the column. The drop panel will contain extra rebar and strengthen the slab around the column. Note the rebar projecting from the top of the reinforced concrete column that will structurally tie the column to the slab. (Photo by Dee Etzwiler) to as a podium. The long-span con- crete structural bays afford flexibility for retail leasing, and the continuous concrete ceiling/floor provides code- mandated fire separation as well as acoustical separation between the commercial and residential uses. Framing The framing of walls, floors, and roof is essentially the same for a relatively enormous multifamily building as for a single-family house. Gravity and lat- eral loads are greater because of the increased height and mass, but the same joists, rafters, trusses, and other framing components are generally adequate. Stud sizes in the lower lev- Figure 22.24 els may need to be increased, and Workers finishing a ground-level structural posttensioned slab over a parking garage. stud spacing may have to be reduced, Notice the one worker operating a power trowel while two riding power trowels sit in order to support the greater num- idle. It is important for the contractor to have sufficient equipment and manpower ber of stories above (Figure 22.25). available to place and finish large concrete projects such as this within a short time In all cases, engineering for lateral frame. Weather and concrete mix are variables that also need to be considered. resistance is critical. Shear walls and (Courtesy of Rowell/Brokaw Architects) hold-downs must be stronger than  Chapter 22 Multifamily Construction / 531 those required for single-family hous- es, but wood frame construction up to six stories tall has been shown to be structurally adequate for even the most restrictive seismically active zones (Figure 22.26). Until very recently, many devel- opers refused to build multistory, multifamily housing projects of wood. This was because of the fact that wood changes dimension with a change in moisture content. With all of the cross-grain structure stacked up in a multistory building, the overall height of the building could change a couple of inches over the course of a year. This dimensional change has caused untold amounts of cracking plaster and envelope failures, and building owners have lost much of their profit to repair bills. Fortunate- ly, the advent of the engineered floor Figure 22.25 structure has appreciably changed This framed wall is on the first floor of a three-story multifamily building. The framing this situation. The substitution of is essentially the same as that for a single-family residence except the lateral loads are stable, engineered floor structure for great enough that the bottom plate needs to be a 4 × 6 to allow sufficient nailing at the dimension lumber has eliminated base of the shear walls. (Courtesy of Rowell/Brokaw Architects) 80 percent of the cross-grain lumber from the system, and owners and de- velopers can now be confident in the stability of four- and five-story wood structures. The most significant departure from standard framing for a multi- story, multifamily building usually occurs at the floor between the low- est level of residential occupancy and the parking or commercial level be- low. This is the same floor level where structure is often supplied by a post- tensioned slab. In this situation, when framing is used instead of a concrete slab, large beams made of steel, concrete, or engineered wood must be employed because of the need for long spans (Figure 22.12). The deep beams can make it difficult to route plumbing and ductwork, so it is im- portant to plan carefully for these sys- tems when a beam system is used. Figure 22.26 Other framing practices typi- This six-story, 23-unit, wood-framed apartment building was tested with a 7.5 magnitude cal for multifamily buildings include simulated earthquake on a monster shake table. The test, which took place in Japan walls between units that are framed during July 2009, reproduced the motion and 180 percent of the force of the devastat- for acoustical separation and some ing Northridge earthquake. The test produced only minor damage to the building in the special framing to achieve fire separa- form of cracked drywall and withdrawn nails. (Courtesy of Simpson Strong-Tie Co., Inc.) tion (Figures 22.27 and 22.28). 532 / Part Four Alternative Construction Systems Wall description and estimated STC rating Figure 22.27 One layer of 5/8′′ Type X gypsum A chart showing a few of the numerous board on each side of 2 × 4 wood wall assemblies available to designers studs @ 16′′ o.c. trying to achieve acoustical separation No insulation: STC — 35 between adjacent dwelling units. Tables With insulation: STC — 38 such as these are based on scientific testing commissioned by manufacturers of One layer of 5/8′′ Type X gypsum the materials in the assemblies. board on each side of 2 × 4 wood studs @ 16′′ o.c., with resilient channel on one side. No insulation: STC — 40 With insulation: STC —47 Two layers of 5/8′′ Type X gypsum board on each side of 2 × 4 wood studs @ 16′′ o.c. No insulation: STC — 43 With insulation: STC — 45 One layer of 5/8′′ Type X gypsum board on each side of staggered 2 × 4 wood studs @ 16′′ o.c., on 2 × 6 plates. No insulation: STC — 46 With insulation: STC — 47 Two 2 × 4 walls spaced 1′′ apart with one layer of 5/8′′ Type X gypsum board on each side of framing. No insulation: STC — 56 With insulation: STC — 59 Ceiling description and estimated STC rating Figure 22.28 Flooring over 3/4′′ plywood subfloor, A wide range of floor/ceiling assemblies on top of 2 × 10 wood joists @ 24′′ o.c. are available to achieve acoustical separa- Ceiling finish is 5/8′′ Type X tion between levels. gypsum board. STC rating: 37 Flooring over 3/4′′ plywood subfloor, on top of 2 × 10 wood joists @ 24′′ o.c. Ceiling finish is 5/8′′ Type X gypsum board attached to resilient channel. STC rating: 45 11/2′′ gypsum concrete over 3/4′′ plywood subfloor, on top of 2 × 10 wood joists @ 24′′ o.c. Ceiling finish is two layers of 5/8′′ Type X gypsum board attached to resilient channel. STC rating: 58  Chapter 22 Multifamily Construction / 533 Acoustical Separation Units within a multifamily building must be designed for the highest practical level of acoustical separa- tion. Walls and floors must be de- signed to absorb both airborne noise (like music and speech) and struc- ture-borne (impact) sounds. Stan- dard details for floors and walls with reduced sound transmission are pub- lished, to meet building code require- ments for minimum levels of acousti- cal control between living units in the same building. For airborne sound, for example, a standard 2 × 4 wall with ½-inch drywall on both sides has a Sound Transmission Class (STC) rat- ing of 33. At this level, loud speech is barely intelligible from the opposite side of the wall. By adding extra mass in the form of extra layers of drywall and adding sound-absorbing insula- tion within the wall cavity, an STC rat- ing of 43 can be achieved. A further Figure 22.29 reduction may be made by mount- Workers apply gypsum underlayment that is pumped through a hose and distributed ing the gypsum board on resilient with a straightedge tool with tiny legs to keep it a constant distance above the subfloor. sheet metal channels that damp the (© Gyp-Crete Corporation, Hansel, Minnesota) transmission of vibration by the wall structure. Building codes typically decoupling of structural layers—strat- carefully. A number of products have specify an STC level of 50 for walls be- egies that work to control airborne been introduced to the market to tween units, a rating that can only be sound—are also effective. In high- deal with this problem. The perim- achieved by separating the structure quality residences, a thin (1½ inch) eter of the wall must be made airtight on the two sides of the wall with stag- layer of low-density, self-leveling liq- with acoustical sealant. gered studs or double walls. An STC uid gypsum is usually added over the rating of 50 is sufficient to damp even subfloor before carpet or resilient sounds such as a loud stereo to a level flooring is laid (Figures 22.29 and Fire Protection that most tenants will find acceptable 22.30). A layer of cushioning material (Figure 22.27). is usually installed below hardwood, Considerable time and effort are spent Structure-borne sounds are usu- tile, or other flooring material with a in controlling losses due to fire in ally transferred through floors by the hard surface. Batt insulation installed multifamily buildings. Building codes impact of footsteps or children play- in joist cavities is effective and usually contain many measures designed to ing, but can also be caused by such necessary, and resilient channels can eliminate sources of combustion, con- activities as chopping vegetables or be used on the underside of ceiling trol the combustibility of the building working on projects at counters or joists to separate the drywall from the and its contents, contain the fire with- desks. Similar to walls, sounds through structure (Figure 22.31). in a very limited portion of the build- floor/ceiling assemblies are typically A wall with a high STC can be ing, extinguish the fire automatically, controlled with a combination of made ineffectual by even a small and facilitate rapid, safe evacuation of strategies (Figure 22.28). Cushioning penetration like an electric box, a the building if a fire should occur. the structure with soft materials such lighting fixture, or an air leak under During the formative stages of a as carpet is the easiest way to mini- the sole plate. Electrical outlets, for multifamily project, before the design mize the transmission of impact noise, example, are a frequent path for un- is established, discussions about fire- but the introduction of mass and the wanted sound, so they must be sealed protection strategies inevitably occur 534 / Part Four Alternative Construction Systems Figure 22.30 A 1½-inch self-leveling nonstructural gypsum slab has been poured on top of a plywood subfloor to help reduce sound transfer through the floor. In preparation for this, the walls are built with a double bottom plate so that 1½ inches for nailing remains above the gypsum slab. The gypsum slab also contributes to fire resistance. (Courtesy Rowell/Brokaw Architects) Figure 22.31 Resilient channel and sound-absorptive insulation installed on the ceiling of an apartment with another living unit above. The resilient channel will help to isolate from the structure the gypsum board that will be screwed to it. Together, the resilient channel and insulation will work with a gypsum slab and carpet above to minimize sound transfer through the ceil- ing/floor assembly. (Courtesy of Rowell/ Brokaw Architects) among the fire marshal, the building example, the use of fire-resistant-treat- the combustibility of interior finish official, the owner, the architect, and ed framing (Type III-B construction) materials and to control their use. the engineer. What type(s) of sprin- can eliminate the need for fire-rated Several catastrophic fires were attrib- kler system will be employed? Will assemblies between units. uted to highly flammable ceiling tiles fire-resistant-treated framing lumber around the middle of the 20th cen- be used? The outcome of these discus- tury, so the fire spread characteristics sions will have a significant impact on Regulating Combustibility of such materials are now regulated the cost and will set the direction of To reduce the risk of a fire in the first by code. To control combustibility how the building will be detailed. For place, it is important to understand of finish materials, every material is  Chapter 22 Multifamily Construction / 535 tested and assigned a flame-spread Figure 22.32 rating (ASTM E84) in relation to the Fire sprinkler con- known flame-spread characteristics trols in the stairwell of asbestos board (flame spread 0) of a 5-story apartment and dry red oak (flame spread 100). building under con- Tested materials are assigned to one struction. The vertical of five classifications, and only those 6-in. steel trunk line in the highest two categories are al- supplies water under lowed for use in large areas of a pressure to all floors building. and also has a branch Class A: flame spread 0–25 line to the street, Class B: flame spread 26–75 where a pumper truck can connect. The Class C: flame spread 76–200 small-diameter steel Class D: flame spread 201–500 pipe at right supplies Class E: flame spread 501 and above sprinkler heads at the parking garage level. Sprinkler heads at the residential levels are Fire Sprinkler Systems connected with plastic Most large multifamily residences are piping. (Photo by Rob equipped with fire sprinkler systems. If Thallon) not mandated by code, designers usu- ally specify a sprinkler system because its inclusion allows other fire-related code requirements to be reduced. For example, the required fire separation between units can be reduced from a 2-hour rating to a less complicated and less expensive 1-hour rating. Even if the overall first cost of a building that, when connected to a pressurized common construction assemblies and will be more with a sprinkler system, water source in the event of a fire, components can be found in the IRC the owner will usually want it because they deliver large volumes of water to and the IBC and in a variety of cata- insurance rates will be lower and upper floors, eliminating the need to logs and handbooks issued by building property and lives will be safer. route fire hoses through stairways. material manufacturers and organiza- Sprinkler systems in multifamily tions concerned with fire protection of buildings are generally stand-alone buildings (Figure 22.33). In each case, Fire Separation the ratings are derived from full-scale systems that do not interact with the potable water supply system (Fig- The building codes require that mul- laboratory fire tests carried out in ac- ure 14.19). This means that pres- tifamily buildings be constructed so cordance with ASTM E119. In general, surized water stands in the sprinkler that a fire cannot pass easily from one a 1- or 2-hour rating will be required pipes for long periods of time, and unit to another or between other im- between units in a multifamily build- a check valve must be installed to portant parts of the building such as ing, depending on what class of sprin- prevent backflow of sprinkler water a parking garage and an exit hallway. klers is included and whether a wall is into the potable source. Piping size To accomplish this isolation of parts of a bearing wall or a partition wall. All depends on the size of the system the building, the codes describe wall required exits must be protected with and the available water pressure (Fig- and floor assemblies that are rated 2-hour assemblies (Figure 22.34). ure 22.32). To be effective, sprinkler in relation to their resistance to the heads must be located at all egress passage of a fire from one side to the other. Fire-resistance ratings are given Fire Blocking routes and within units in all signifi- cant rooms. Dry standpipes are also in hours—a 1-hour wall, for example, Fire blocking in the framing is an inte- sometimes employed in multifamily will resist fire penetration for 1 hour gral part of the fire-resistance system. buildings. A standpipe system consists when tested in a standardized furnace, Blocking between floors and between of large empty pipes incorporated while a 3-hour wall will resist for 3 units is essential to stop the flow of into the structure of a building such hours. Fire-resistance ratings of many fire that might find its way through 536 / Part Four Alternative Construction Systems Wall description and estimated fire rating 1 - hour wall Figure 22.33 5/8′′ Type X gypsum board (both sides) Fire-resistance rating chart. 2 × 4 wood studs @ 16′′o.c. 3 ′′ mineral wool batt plaster finish (both sides) 2 - hour wall (2) 5/8′′ Type X gypsum board (both sides) 2 × 4 wood studs @ 16′′o.c. 3′′ mineral wool batt plaster finish (both sides) Figure 22.34 Fire stairs must be protected by walls with a 2-hour fire-resistance rating. In this example, two layers of 5/8-inch drywall with offset joints have been applied to the framing before the stairs were constructed. (Photo by Dee Etzwiler) framing cavities. Any penetrations provide service to the entire building through the framing for electrical, Systems (Figures 14.17, 22.36, and 22.37). plumbing, and other systems must be The mechanical, electrical, and With respect to mechanical sys- sealed with specially formulated com- plumbing systems (MEP systems) for tems, there is a difference between pounds in order to satisfy fire sepa- large multifamily buildings are essen- buildings occupied for extended ration requirements (Figure 22.35). tially larger versions of single-family periods by owners or by renters and Any ductwork that passes through a systems. While the MEP systems with- those with short-term or group rent- fire-resistive assembly is required to in each unit of a multifamily building als. The former typically employ have an approved fire damper that are virtually the same as those found residential-scale HVAC systems for will close automatically if hot gases in a single-family detached version, each unit, while the latter often have from a fire should enter the duct. the collection of many units in one large centralized systems. The dif- Because of all the requirements, building requires a scale of systems ference is driven by the need to bill fire-resistive assemblies are relatively that is distinct. Connections to all owners or long-term renters directly expensive in relation to their gener- municipally supplied services such as for their utility use. For example, ic counterparts, so the efficiency of potable water, electrical power, natu- condominiums and apartments have their design is very important. ral gas, and sewer must all be sized to residential-scale mechanical, electri-  Chapter 22 Multifamily Construction / 537 Figure 22.35 All penetrations for utilities that occur between units must be sealed with an ap- proved compound to achieve a required 1-hour fire rating. In this case, the fire barrier occurs at the ceiling level, and the sealed penetrations are in the double top plate. (Photo by Rob Thallon) cal, and plumbing systems at each unit, and each is metered. For the convenience of the utility company, meters are usually consolidated into one area of the building (Figure 22.38). In hotels and dormitories, it is usually most efficient to have large centralized systems for hot water and sometimes for space heating/cool- ing. In these systems, the problem is distribution. Hot water can be con- stantly circulated, making it imme- diately available at every tap. Where rooms are heated with a centralized system, hot water is usually the vehi- cle to carry the heat because the pip- ing to carry it can go long distances while consuming very little space. Acoustical issues related to sys- tems are much more critical in a multifamily building than in a single- family residence. It is much more annoying to hear your neighbor’s plumbing, for example, than that of your own family. Plumbing supply pipes can be isolated from framing, they can be insulated, and quiet valves can be specified. Plumbing drains, when they are adjacent to another unit, are usually made with cast iron piping, which, because of its density and its soft rubber joints, is much qui- eter than plastic (Figure 22.39). Heat- ing systems can also be noisy, so care must be taken with their specification Figure 22.36 and installation. Electrical outlets on Most of the electrical and communication cables for this 49-unit apartment building walls between units must be carefully under construction originate from this location. All utilities must be installed and sealed to minimize the passage of air- bedded in gravel before the slab can be poured. (Photo by Rob Thallon) borne sounds. 538 / Part Four Alternative Construction Systems Figure 22.37 Plumbing waste lines from residential units above are seen passing through a ground-level commercial space under construction. Note that the plumbing is made of cast iron and the framing is light-gauge steel, both required in commercial construction. (Courtesy of Rowell/Brokaw Architects) Figure 22.38 Located in an alley where most utilities originate, this collection of plumbing, electrical, communication, natural gas, and fire sprinkler systems will have easy access for meter reading and service. (Photo by Dee Etzwiler)  Chapter 22 Multifamily Construction / 539 Figure 22.39 Cast iron waste piping is much quieter than plastic piping, so it is often employed between units of multifamily housing. Notice that the pipe transitions to less expensive plastic piping when it is positioned away from the party wall. (Courtesy of Rainbow Valley Design and Construction) Accessibility According to the Americans with Disabilities Act (ADA) Guideline 4.1.3(5), buildings that are three or more stories tall are generally re- quired to have an elevator. Multifam- ily residential buildings up to five sto- ries tall are typically equipped with a hydraulic elevator—the least expensive of the options (Figure 22.40). A stan- dard hydraulic elevator operates by means of a vertical cylinder sunk into the ground to a depth that is as great as the vertical travel of the elevator. A plunger in the cylinder pushes the elevator car up when hydraulic fluid Figure 22.40 is pumped into the cylinder. The el- Diagram of a typical plunger-type hydraulic evator descends by gravity, forcing elevator installation. The pump, motor, and oil the hydraulic fluid out of the cylinder tank are located in a small machine room, sepa- into a tank. A telescoping plunger rate from the carriageway. (Courtesy of Schindler can reduce the depth of the cylinder. Elevator Corp.) 540 / Part Four Alternative Construction Systems The construction of a hydraulic elevator spans more of the construc- tion schedule than almost any other element because typically the hole must be drilled in the ground and the cylinder and piston installed before the foundation is poured. The elec- tronics, doors at each floor, and fin- ish of the car are not completed until near the end of the project. Between these times, the elevator carriageway is built and fire-protected by subcon- tractors working on the foundation, framing, and drywall. It is common to construct the carriageway of con- crete or concrete masonry at its base and of wood at the upper residential floors. In any case, it must be fire rated to a 2-hour standard. When the carriageway is complete, the elevator subcontractor installs the guide rails, assembles the car, connects the pump and controllers, and puts the doors in Figure 22.41 place at each floor. The coordination of materials, workers, and machinery on a constrained urban site Hydraulic elevators currently requires careful planning. The construction schedule is especially important for com- dominate the low- and midrise mar- mercial multifamily project such as this where owner income is tied to a completion kets because they are less expensive date. (Photo by Rob Thallon) than other types, they are compact, and their structural requirements are minimal. They do have some drawbacks, however. They require a fire stair shafts are usually construct- required to incorporate “just-in-time” separate machine room for the pump ed of reinforced concrete, making a delivery into their bids. The selection unit, tank, and control system, and strong shear wall in each direction of of the construction system itself may they are noisy and slow compared the building and serving to brace the well be affected by site constraints. to other elevator types. A relatively open story. For example, the steel frame system recent alternative to the hydraulic el- shown in Figures 22.13 and 22.14 was evator is the machine room–less (MRL) chosen instead of a posttensioned elevator, which is more energy effi- Site Management slab for a very constrained site be- cient and does not require a space- cause the contractors could use the consuming machine room or a deep High-density housing is often built space in the parking garage as soon cylinder well. in an urban environment where the as the framing was erected—whereas Multifamily buildings with units building itself will occupy virtually they otherwise would have had to wait on two or more floors are required the entire site. This condition makes weeks while the space was cluttered to have two means of egress (escape) it very difficult to manage all the with a forest of structural slab shoring in case of fire or other disaster. This equipment, materials, and workers (Figure 22.22). requirement usually results in two necessary to complete construction A small site also can make stan- separate fire stairs that are enclosed (Figure 22.41). To make the project dard tasks take longer because there with 2-hour-rated walls and fire doors feasible, the general contractor may is no room to accomplish them ef- (Figure 22.34). Because the stairs have to extend the site by securing a ficiently. Setting up a crane, for ex- have solid vertical walls and must gen- public right-of-way permit to occupy ample, can take much more time in erally be located at opposite ends of the sidewalk and/or street or by rent- tight quarters because each section of the building, they are often designed ing space from neighboring private the boom arm must be individually so that their walls contribute to the properties. Deliveries of materials hoisted into position and bolted to lateral stability of the building. At to the site must be carefully orches- the crane in the air rather than being parking or commercial floor levels, trated, and material suppliers may be preassembled on the ground.  Chapter 22 Multifamily Construction / 541 The height of multifamily build- to the adjacent food market so as to that units are stacked and therefore ings usually requires that scaffolding avoid excessive truck traffic in the consume less site area and fewer or cherry pickers be employed to fin- area at any one time. Thoughtful building materials is significant. ish the exterior walls (Figure 22.42). contractors will keep their neigh- Heating and cooling a unit sur- The scaffolding is affected by the sid- bors informed of progress by dis- rounded on several sides by other ing and exterior details and is a signifi- tributing a weekly construction up- units requires less energy than the cant cost factor in designing the ex- date and may even adjust their work same size unit standing alone. Mul- terior of the building. If the building schedule to minimize or eliminate tifamily units are typically smaller can be detailed so that the exterior fin- construction noise during certain than single-family units, and they ishes are installed on the wall frames hours. are usually in an urban environ- before they are tilted up, this cost can ment where walking and public be largely avoided (Figure 22.43). transport are reasonable options In urban settings, the delivery (Figure 22.45). (It is said that resi- of materials must often be coordi- Green Issues for dents of New York City consume the nated with other commerce in the Multifamily Housing fewest resources per capita of any area. The concrete deliveries for place on the continent.) the project shown in Figure 22.41 Inherently, multifamily housing has The size of the entire multifam- were synchronized with the deliver- great potential to be the most green ily building often allows green strate- ies of produce and other groceries of all housing types. The simple fact gies to occur because of an economy Figure 22.42 Fixed scaffolding was used on this apartment building because workers had to spend hours applying a stucco surface to the walls. Sophisticated, adjustable scaffolding for buildings of this scale is available in a variety of forms. (Courtesy of Rowell/ Brokaw Architects) 542 / Part Four Alternative Construction Systems Figure 22.43 A three-story student apartment complex built over a below-grade parking garage. The top floor walls were framed com- plete with windows and painted siding to save on the cost of scaffolding. (Photo by Rob Thallon) Figure 22.44 Workers here are installing prefabricated wall panels at an upper level of a multi- family housing project. Off-site prefabri- cation is a common strategy to speed the construction process and minimize the problems of storing materials and laying out framing on a sm