Technical Assignment #3 November 15, 2004 Lateral System Analysis and Confirmation Design

Transcription

1 Executive Summary Technical Assignment #3 November 15, 2004 Lateral System Analysis and Confirmation Design The Food Science Building in State College, Pennsylvania is a four story above ground steel composite structure with grade beam and pile foundation. It also has basement and penthouse levels. The building foundation is composed of grade beams, piles, and pile caps. Its main structural elements include a lightweight concrete slab on steel beams with moment frames. This report analyzes the lateral bracing for The Food Science Building. The building is framed by moment frames in both the longitudinal (EW) and latitudinal (NS) direction. Using the data from technical report #1, seismic lateral loads are applied to the moment frames. Drift, deflection, torsion, and strength are all calculated using RAM. After analyzing the data, it was found that the moment frames can sufficiently carry the full lateral loads within a deflection limit of H/400. 1

2 Existing System The existing lateral system in The Food Science Building consists entirely of moment frames in both directions. On the first floor level the exterior loads are transferred to grade beams and then to the piles and pile caps. The interior loads are transferred by concrete encased columns directly to the piles and pile caps. All interior, non-load bearing masonry walls exceeding the height of 10-0 and not over a grade beam shall bear on a thickened slab. In most cases the frames start at the second level and continue up to the full height of the building. For simplification reasons, the drift, overturning, (etc ) calculations were tabulated as if the moment frames continued throughout full height of the building. Two 2-D frames are pictured below. The three dimensional frame was created using RAM. Figure 1: West-East framing of the entire building. Figure 2: South-West framing of the entire building 2

3 Figure 3: Seen here are the major framing elements of a typical bay. All of the members shown in red are steel moment frames. Other members are gravity members only. Lateral Load Path Lateral Load Exterior Walls Composite Slab Beams/Girders Concrete Encased Steel Columns Pile Caps Piles Ground 3

4 Distribution of Lateral Loads The lateral loads were determined based on the wind and seismic calculations from the Structural Concepts/Structural Existing Report. They were distributed according to rigidity. All frames were modeled using RAM. Floor Plan West, Shaded area shows the analyzed typical bay Wind Loads The wind loading was calculated using the same procedure as technical assignment 1. Please refer to technical assignment 1 for the extensive wind load calculations. The L-shaped building was used in calculating the wind load calculations. Wind pressure in both the North-South direction as well as the East-West direction was calculated. When rounded to the nearest number, the North-South and East-West pressures were identical. Therefore, the load per foot over is the same for both sides of the bay. Wind Load has been calculated based on the following: 3-second gust, 90mph Importance factor (1) : 1.0 Exposure- B Maximum Windward Pressure = psf Maximum Leeward Pressure = 7.71 psf 4

5 Figure 4: East-West Windward Pressure Figure 5: North-South Windward Pressure Seismic Figure 6: Load per floor over the entire side The seismic base shear was calculated using the Equivalent Lateral Force Procedure for the L-Shaped building only; this is the same procedure used in technical assignment 1. Please refer to technical assignment 1 for the extensive 5

6 seismic base shear calculations. A roof dead load of 33 psf, snow load of 36 psf, floor dead load of 85.5 psf and an exterior wall load of 40 psf was used. The roof dead load accounts for the deck, insulation, roofing, and miscellaneous dead load. When the forces were distributed to the individual frames it becomes clear that the seismic forces take precedence over the wind loading and will therefore govern the lateral design. Seismic Load has been calculated based on the following: Site Class: D Site-Adjusted Spectral Response Acceleration for 0.2 second pd. (Sms): 0.272g Site-Adjusted Spectral Response Acceleration for 1.0 second pd. (Sms): 0.144g Seismic Hazard Exposure Group 1 Seismic Use Group: II (table 9.1.3) Vertical Distribution of Seismic Forces: Level w x h x w x h x (roof) w x h x C vx (N- S) C vx (E- W) F x (N- S) F x (E- W) Σ= Σ= Σ= Σ= Σ= Σ= Figure 7: Load per floor over the entire North-South Side 6

7 Drift Figure 8: Load per floor over the entire East-West Side A model was run in RAM to determine drift, deflection, overturning, and to check strength. Story drifts were calculated using the seismic loads with a total story drift of 6.46in. Each story drift is given below. Level Story Drift Penthouse 3.11 I am not anticipating these drifts to be accurate due to the fact that only a portion of the building was analyzed and taken as a separate body. According to IBC 2003, allowable story drift for a building in Seismic Use Group II is 0.015h sx. The total drift equates to 2.37, which is less than 2.40 (the standard practice building drift of h/400 from the Gaylord and Gaylord Structural Engineering Handbook would result in a max drift of 2.40.) The allowable drifts are given below. Level Story Drift Penthouse.16 7

8 Overturning/ Foundations The controlling seismic load case was applied to the North/South direction. Uplift was created at the exterior column footing and the interior column. The dead load of the building is adequate enough to resist the 9,385 ft-kip overturning moment. Strength/Design There was no failure found in the RAM model of the beams and columns. They were adequate to carry the calculated loads. Story drift on each level was determined to be within the proposed h/400 for seismic loading. The uplift and overturning calculations justify the current foundation plan. Conclusion The moment frames have proven to be an effective solution to the wind and seismic loads which will be encountered by The Food Science Building. The exterior cladding will absorb the wind or seismic loads and transport through to the slabs. The moment frames will then receive the lateral load from the slabs. The overall drift for the structure was found to be under the standard practiced building drift of H/400. The drift calculated by the wind force proved to create unacceptable drifts. This might be due to the fact that my analysis was done on a single isolated beam, not the entire building frame. 8

9 APPENDIX Criteria, Mass, and Exposure Data..10 Frame Model Data (Please see the appendix in technical assignment #1 for the wind and seismic loading calculations.) 9

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