Design Example 2 Flexible Diaphragm Design

Transcription

1 Design Eample Fleible Diaphragm Design OVERVIEW This eample illustrates the design o a large leible diaphragm in a big-bo retail store subjected to lateral seismic loading. The roo structure consists o a panelized hybrid roo system, which is very common in large-diaphragm roos in the seismically active western United States. This roo system comprises structural wood-panel sheathing with light dimensional lumber supports, resting on open-web steel joists and joist-girders. While this eample illustrates the design o a wood diaphragm, a similar methodology is applicable to untopped steel deck diaphragms. OUTLINE 1. Roo Diaphragm Lateral Loading. Shear Nailing o the Roo Diaphragm (North-South) 3. Considerations or Plan Irregularities 4. Diaphragm Chords (North-South) 5. Diaphragm Collectors 6. Diaphragm Delection 015 IBC SEAOC Structural/Seismic Design Manual, Vol. 19

2 Figure -1. Typical building with leible diaphragm IBC SEAOC Structural/Seismic Design Manual, Vol.

3 Figure -. Eample s roo plan Figure -3. Eample s building section 015 IBC SEAOC Structural/Seismic Design Manual, Vol. 131

4 Given Inormation Seismic-orce-resisting system Bearing-wall system consisting o intermediate precast concrete shear walls supporting a leible diaphragm o wood structural panel. Seismic and site data Mapped spectral accelerations or the site S = 1.5 (short period) S s 1 = 0.6 (1-second period) Risk Category = II (Occupancy) Site Class = D Seismic Response Coeicient R = 4 (T 1.-1) V = 0.5W (ASCE 7 Section 1.8.1) Seismic Design Category = D S DS = 1.0 Wind Assumed not to govern Roo Dead load = 14 ps Live load (roo) = 0 ps (reducible) (IBC Table ) Walls Thickness = 7.5 inches o concrete Height = 3 eet Normal weight concrete = 150 pc Roo Structure Structural-I sheathing (oriented strand board wood structural panel) Pre-engineered/pre-manuactured open-web steel joists and joist-girders with ull-width wood nailers. All wood is Douglas-ir. 1. Roo Diaphragm Lateral Loading 1.1 ROOF DIAPHRAGM SHEAR COEFFICIENT The roo diaphragm must be designed to resist seismic orces in each direction. The ollowing ormula is used to determine the total seismic orce F p on the diaphragm at a given level o a building. F p = n i= n F i wp. ASCE 7 Eq w i= i IBC SEAOC Structural/Seismic Design Manual, Vol.

5 As given, the base shear or this building is V = 0.5W. Because it is a one-story building, Equation simply becomes the ollowing: E D+L L+ S F p shall not be less than = (.0 ) ( ) 0. S DS I e w p ).0 (1. 10 w p = 0. wp ASCE 7 Eq but need not eceed 04.4 S DS Ie w p w p 04w.4 wp. = ( )( ) = ASCE 7 Eq Based on the criteria given in Section , F p = 0.5w p. Thereore, or diaphragm design use F p = 0.5w p. 1. ROOF DIAPHRAGM SHEARS The wood structural panel roo system is permitted to be idealized as a leible diaphragm per ASCE 7 Section or SDPWS Section Seismic orces or the roo are computed rom the tributary weight o the roo and the walls oriented perpendicular to the direction o the seismic orces. Walls parallel to the direction o seismic orces do not load the leible diaphragm. The distributed lateral loading to the diaphragm will be computed in each orthogonal direction. East-west direction Because the panelized wood roo diaphragm in this building is idealized as leible, lines A, B, and E are considered lines o resistance or the east-west seismic orces. A collector is needed along line B to drag the tributary east-west diaphragm orces into the shear wall on line B. The loading and shear diagrams are shown in Figure -4. Figure -4. East-west diaphragm loading 015 IBC SEAOC Structural/Seismic Design Manual, Vol. 133

6 The uniorm loads W 1 and W in the east-west direction are computed using the diaphragm lengths and wall heights. Roo dead load = 14 ps 75. Wall dead load = pc ps = Roo height = 1 eet average Parapet height = eet average. 3 1 W 1 = 05( ( 9 )( 3).51 (14 ps )( )(40 ) t)+ ) 05( 0. (906. ps = 1411 pl W = )( 3).5 (11 4 ps )(30 t) + ( 06p s = 1691 pl. In this eample, the eect o any wall openings reducing the wall weight has been neglected. This is considered an acceptable simpliication because the openings usually occur in the bottom hal o the wall. In addition, signiicant changes in parapet height should also be considered i they occur due to signiicant roo slope. Diaphragm shear at line A and on the north side o line B is 8, 00 lbs = 118 pl. 40 t Diaphragm shear at the south side o line B and at line E is 101, 000 lbs = 316 pl. 30 t North-south direction Diaphragm orces or the north-south direction are computed using the same procedure and assumptions as the east-west direction and are shown in Figure -5. W = ) )( 3 ).5 (11 4 ps )(10 t) t + ( 06p s W 3 = 991 pl W = (11 4 ps )( 160 t) ) (. ps 3 = ( ) T ( ) K T (1 )( ) ( )( ) Diaphragm unit shear at line 1 and the west side o line 3 is 39, 600 lbs = 330 pl. 10 t IBC SEAOC Structural/Seismic Design Manual, Vol.

7 Figure -5. North-south diaphragm loading Diaphragm unit shear at the east side o line 3 and at line 9 is 136, 000 lbs = 850 pl. 160 t. Shear Nailing o the Roo Diaphragm (North-South) The diaphragm loaded in the north-south direction has been selected to illustrate the design o a wood structural panel roo diaphragm. A similar design is required in the other orthogonal direction, east-west, but is not illustrated here. Allowable stress design (ASD) will be used. The basic loading combinations are given in IBC Section , and those involving earthquake loading have been simpliied in ASCE 7 Section The governing seismic load combination or allowable stress design is (5) ( S DS )D + H + F + 0.7ρQ E IBC SEAOC Structural/Seismic Design Manual, Vol. 135