BT5 P3-Reviewer (2024) Thin Shell Structures PDF

## BT5 P3- REVIEWER(2024) ### CHAPTER 6 ### THIN Shell structures #### SHELL STRUCTURES- - A shell structure is a thin, curved membrane or slab, usually of reinforced concrete, that functions both as structure and covering, the structure deriving its strength and rigidity from the curved shell form...

## BT5 P3- REVIEWER(2024) ### CHAPTER 6 ### THIN Shell structures #### SHELL STRUCTURES- - A shell structure is a thin, curved membrane or slab, usually of reinforced concrete, that functions both as structure and covering, the structure deriving its strength and rigidity from the curved shell forms. - Shell structures predominantly resist loads on them by direct compression. That is without bending or flexure. - Since most materials are more effective in compression than in bending, shell structures result in lesser thickness than flat structures. - Is a structure composed of a relatively thin shell of concrete usually with no interior columns or exterior buttresses. The shells are most commonly flat plates and domes, but may also take the form of ellipsoids or cylindrical sections, or some combination thereof. #### Natural shells - The term "SHELL" is used to describe the structures which possess strength and rigidity due to it's thin, natural and curved form such as shell of egg, a nut, human skull, and shell of tortoise. #### advantages - Concrete shells are naturally strong structures - Allowing wide areas to be spanned - Reduces both material cost and construction cost #### Disadvantages - Issues with sealing - Prevents air from escaping - Labor cost #### TYPES - The term single and double curvature are used to distinguish the comparative rigidity of the two forms and complexity of centering necessary to construct the #### Developable surfaces (single curve) - Is a surface that can be unrolled onto a flat plane without tearing or stretching it. - It is formed by bending a flat plane - The most typical shape of a developable shell is a barrel and a barrel shell is curved only in one direction. #### BARREL- - Is a combined Arch action and Beam action. And there are mainly two types of barrel; - long barrels, arch action is prominent - short barrels, beam action is prominent #### LONG BARREL SHELLS - These are typically supported at the corners and can behave structurally as a large beam #### LONG SPAN BARREL VAULT - Span is larger than its width. - Strength of a structure lies at the right angles to the curvature to that span is longitudinal to the curvature #### SHORT BARREL SHELLS - These shells are typically supported at the corners and can behave in one or a combinations of different ways #### SHORT SPAN BARREL VAULTS- - The span is shorter than it's width of the arch ribs between which the barrel vault span #### Double Curvature Shells- - Double curvature shells are either part of a sphere, as a dome, or a hyperboloid of #### shell form - Single Curvature Shells- Single curvature shells, curved on one linear axis, are part of cylindrical or cone in the form of barrel vaults and conoid shells. #### MAIN CLASSIFICATION: - Synclastic shells - Anticlastic shells #### Synclastic shells - are double curved and have a similar curvature in each direction. - a dome is a good example of a synclastic shell, it is double curve and can be performed by rotating a curved line around an axis - similar to arches under a uniform loading the dome is under compression everywhere, and the stresses act along the arch and hoop lines. #### anticlastic shells - Double curved but each of the two curves have the opposite direction to the other - Conoids, Hyperbolic Paraboloid and Hyperboloids are all considered to the anticlastic shell because they are saddled shape with different curvature in each direction and straight lines can be drawn of the surface - Depending on the shape of the shell relative to the curvature, there will be different stresses - Shell roofs, have compression stresses following the convex curvature and the tension stresses follow the concave curvature. #### Z shell - Each of the units above has one large sloping plate and two edge plates arranged with space between the units for windows. - The windows are normally open to the north but most of the light is actually reflected south light. - Adjacent units should be tied together by structural window mullions. - In constructing the Z shell, movable forms need only be lowered a short vertical distance if construction is started on the right and proceeds to the left. - The Z shell is not an efficient structural shape since it is discontinuous and its effective depth is much less than the actual vertical depth. - Therefore, the spans are limited in comparison to the plates having a large number of units side by side. #### Other types of shell structure ##### FOLDED PLATES - Folded plates are the simplest of the shell structures. - The distinguishing feature of the folded plate is the ease in forming plane surfaces. - They are more adaptable to smaller areas than curved surfaces. - A folded plate may be formed for about the same cost as a horizontal slab and has much less steel and concrete for the same spans. - For widths of plate over, say, 12 feet, the thickness of the folded plate must be thicker. - Some advantage may be gained by increasing the thickness of the slab just at the valleys so it will act as a hunched beam and as an I section plate girder.' #### FOUR (4) BASIC ELEMENTS 1) THE INCLINED PLATES 2) EDGE PLATES which must be used to stiffen the wide plates 3) STIFFENERS to carry the loads to the supports and to hold the plates in line, and 4) COLUMNS to support the structure in the air. - If several units were placed side by side, the edge plates should be omitted except for the first and last plate. - If the edge plate is not omitted on inside edges, the form should be called a two segment folded plate with a common edge plate. #### THREE SEGMENT FOLDED PLATE - A sketch shows a folded plate structure with three segments for each barrel. - The end stiffeners are rigid frames rather than deep girders as in the last example.' - The size of the frames may be reduced by using a steel tie between the tops of the columns. - The depth of the shell should be about 0.10 times the span. - The maximum slope of a plate should not be greater than 40 degrees. #### CANOPIES - It has a four (4) segments folded plate. - Absolutely necessary to have a two element system, a torsion member can be placed in the valley which will carry the unbalanced loads. - Stiffeners can often be hidden on the top surface so they are not in evidence and the shell will appear to spring from the vertical column. - At the wall of the building there should also be a stiffener hidden in the wall construction. - Provision for drainage of the centre valley. #### TAPERED FOLDED PLATES - The height of the shells at the centre of the span is the critical dimension for bending strength. - Therefore, the structure is not very efficient and not suitable for long spans because of the excess height required for the large ends. - Another weak element in this design is the transfer of shear from the small end of the triangular plate to the large end. - If a large number of units are used in each span, the transfer of loads may be difficult. - A folded plate may be used for walls as a thin structural element by casting each plate flat on the floor and grouting the joints full of concrete. - A wall of this type can be made much thinner than a flat wall. #### EDGE SUPPORTED FOLDED PLATES - The usual upturned edge plate can be eliminated and the roof structure can be made to appear very thin if the edge plate is replaced by a series of columns. - The slab between columns must be designed as a beam and it may be convenient to extend the main roof slab as a cantilever canopy. - The vertical columns in the end walls at the crown of the gable takes the reactions of the plates and the horizontal ties may be eliminated. - Wind loads are taken by rigid frame action in the columns and stiffeners. #### FOLDED PLATE TRUSS - There are horizontal ties across the width only at the ends of the building and the structure acts as an edge supported shell. - The thrusts from the triangular crossed arches are carried lengthwise to the ends. - The top chord of the inclined truss is formed by the ridge member. - The bottom chords are the ties at the base of the side gables and the diagonals are formed by the sloping valleys at the intersection of the gables and the triangular plates. - The top longitudinal compression member may require some additional thickness to form a compression member of sufficient size to carry the compression force. - This is truly a space structure and its structural action is not as obvious and, therefore, the architectural appearance is more subtle than the usual shell structure. #### CORRUGATED CURVES - Barrel shells in the form of corrugations may offer structural advantages and may have aesthetic values which make such a roof desirable. - Instead of alternative concave and convex circles of the same radius, the curves may be alternate circles of long and short radius. #### CYLINDRICAL BARREL VAULTS - Barrel vaults are perhaps the most useful of the shell structures because they can span up to 150 feet with a minimum of material. - They are very efficient structures because the use the arch forms to reduce stresses and thicknesses in the transverse direction. - Barrel vaults are essentially deep concrete beams with very thin web members and may be designed as such by the ordinary methods of reinforced concrete. #### ELEMENTS OF BARREL VAULTS - The shell has been allowed to project beyond the edge of the stiffener in order to show the shape of the shell. - Stiffeners are required at columns. - In contrast to folded plates where the thickness is based on the design of a slab element, the thickness of the barrel shell is usually based on the minimum thickness required for covering the steel for fireproofing, plus the space required for three layers of bars, plus some space for tolerance. - Near the supports the thickness may be greater for containing the larger longitudinal bars. - If more than one barrel is placed side by side, the structure is a multiple barrel structure and if more than one span, it is called a multiple span structure. #### Four common types of vault - Barrel Vault - Groin Vault - Rib Vault - Fan Vault #### PRE-ENGINEERED BUILDING ##### Introduction - Technological improvements over the year have contributed immensely to the enhancement of quality of life through various new products and services. One such revolution was the Pre-Engineered Buildings (PEB). - Tailor-made solutions were made using this technology in very short durations. * A recent survey by the Metal Building Associations (MBMA) shows that about 60% of the non-residential low rises building in USA are pre-engineered buildings. - Steel industry is growing rapidly in almost all the parts of the world. The use of steel structures is not only economical but also Eco-friendly at the time when there is a threat of global warming. - Here, “ECONOMICAL” word is stated considering time and cost. Time being the most important aspect, steel structures (Pre-fabricated) is built in very short period and one such example is Pre-Engineered Buildings (PEB). - Concept was first originated in the United States after WWII It consists of a complete steel framed building system. ##### Factors made this period significant for the history of metal buildings: - Vast Improvement of Technology - Introduction of new material and its design - Debut of Computer-designed metal buildings - The name Pre-engineered building was adopted for the following reasons: *The term pre-engineered buildings came into being in the 1960s. - Pre-set methods for connecting and welding (standardized connections). - Utilization of pre-determined stock sizes. - Optimized design, detailing and fabrication, resulting in most economical and fast delivery (reduced engineering time and fabrication time). The buildings were pre-engineered because, like their ancestors, they relied upon standard engineering designs for a limited number of off-the-shelf configurations. #### TYPICAL PRE-ENGINEERED BUILDING ##### CHARACTERISTICS 1. Strong & Cost Effective (All Structural Steel) 2. Short Assembly Period (Delivered on-site) 3. Design Flexible. (Factory Assembled and Design) 4. Lighter Building (30% from conventional) 5. Low Rise (25 to 30 m. eave height) 6. Waterproof (use of special mastic beads, filler strips and trims.) ##### TERMINOLOGIES 1. Base Plate - A plate attached to the base of a column which rests on the foundation or other support, usually secured by anchor bolts 2. Butt Plate - The end plate of a structural member usually used to rest against a plate of another member in forming a connection. Sometimes called a split plate or bolted end plate. 3. Bay - The space between frame centre lines or primary supporting members in the longitudinal direction of the building. 4. CONTINUOUS RIDGE VENT - Series of Roof Ventilators connected to each other located along roof peak line. 5. LEVELING PLATE - A steel plate used on top of a foundation or other support on which a structural column can rest. 6. HOT-ROLLED SHAPES - Steel sections (angles, channels, S-shapes, W-shapes, etc.) which are formed by rolling mills while steel in a semi-molten state. 7. PURLIN STRUT - Additional purlin added at or near intersection of wind bracing members at the rigid frame where a series of wind bracing is required in the roof plane. This strut may or may not be a continuous member throughout the length of the building. 8. RIGID FRAME - A structural frame consisting of members joined together with moment connections so as to render the frame stable with respect to the design loads, without the need for bracing in its plane. #### DIFFERENT TYPES PRIMARY FRAMING SYSTEM 1. CLEARSPAN - Self-supporting and requires no interior columns. The interior is completely open and unobstructed. Required wherever a large open area is needed for maneuverability. 2. ARCHED CLEAR SPAN - The column is an RF column while the Rafter is curved. It has no ridge line and peak. The curved roof rafter is used in for aesthetic look. The maximum practical is up to 100meters, but can be extended to 120 meters. 3. STRAIGHT COLUMN - Interior finishing is easier to install than with a tapered column. The straight walls maximize usable floor space. The minimum horizontal loads of a straight column system often save money on slab foundations, as well. 4. TAPERED COLUMN - Provides the widest possible spans without the need for interior supports. They are more economical on larger clear span designs than straight column framing, since building tapered columns use less steel. 5. MODULAR SPAN - Provide a more economical cost per-square-foot at greater widths than clear span framing. Viable option for building applications where minimal interior columns do not pose an operational problem. Provides an affordable alternative for bigger structures. 6. SINGLE SLOPE - Simple and economical both new buildings and additions to existing structures. Practical for adding a wing or other addition to an existing structure. Often this system is used for office space on a large warehouse, manufacturing plant, or vehicle maintenance facility. 7. LEAN-TO - System is a cost-efficient alternative for enlarging a steel-framed building. It is typically used to add space to the side of a steel structure Lean-to framing involves minimal horizontal loads, so foundation costs are often reduced. 8. Multi Span (Ms1) - The Multi spans (MS1) are those which have more than 1 span. The intermediate column is used for the clear span in which width of each span is called width module. 9. Multi Gable - Multi gable has two or more spans where no intermediate columns are used. The columns are added to the extended width 10. Roof Systems - Straight columns with roof having supports. #### STANDARD STRUCTURAL DETAILS - Parapet Fascia - the ridge lines. - Mezzanine Beam Connection to Main Frame Column - and columns are not placed at - Endwall Roof Extension With Soffit #### CHAPTER 7- - Four common types of vault - Barrel Vault - Groin Vault - Rib Vault - Fan Vault - PRE-ENGINEERED BUILDING - Introduction - Factors made this period significant for the history of metal buildings: - Vast Improvement of Technology - Introduction of new material and its design - Debut of Computer-designed metal buildings - TYPICAL PRE-ENGINEERED BUILDING - CHARACTERISTICS - TERMINOLOGIES - DIFFERENT TYPES PRIMARY FRAMING SYSTEM - STANDARD STRUCTURAL DETAILS