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Seismic and Wind Force Calculator

Jonathan Ochshorn

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Directions: Enter general data (city, importance factor), seismic data (site class, seismic force resisting system), and wind data (exposure category, plan and parapet dimensions, and coefficients for directionality and topography). Then, enter values for story heights above grade and seismic weight (approximately equal to the dead load) for each story. Start at the highest floor (i.e., the roof), and work down to the lowest above-grade floor level. Press "update" button.

Story forces for wind and seismic loading will be displayed to the right of the values entered for seismic weight. In this way, the magnitude of wind and seismic forces may be compared for a given building on a given site. Note that there are some limitations for the use of this calculator: the building is assumed to be rectangular, and is limited to 20 stories (for buildings with more stories, an approximate calculation can be obtained by combining the seismic weight of two adjacent stories and entering the average height above grade). Calculations are based on analytic procedures for rigid buildings, neglecting internal pressures (wind), and equivalent lateral force procedures (seismic) as described in ASCE/SEI 7-05, Minimum Design Loads for Buildings and Other Structures. Plan dimensions for wind loading calculations are shown in Fig. 1. To obtain wind story forces from calculated wind pressures, windward and leeward pressures are combined into a single set of forces, as shown in Fig. 2. Account is taken of higher wind pressure on parapets. Story forces for seismic loading are shown in Fig. 3.

More detailed explanations and examples can be found in my text.

sketch showing wind direction and plan dimensions
Fig. 1. Wind direction and plan dimensions.
wind pressure and wind story forces
Fig. 2. Building sections comparing windward and leeward pressures with wind story forces and base shear.
sketch showing seismic forces and base shear
Fig. 3. Building section with seismic story forces and base shear.

floor/roof height above grade (ft) seismic weight per floor (lb) seismic story force (lb) wind story force (lb) windward pressure (psf) leeward pressure (psf)
general data  
city wind speed (mph) seismic Ss seismic S1 seismic TL  
importance factor plan dimensions (ft)  
L B  
wind data  
  parapet height above roof (ft)  
exposure Kd Kt  
seismic data  
lat-force-resist system: main category  
site class (soil)  
paramenters for calculation of period CT x T (sec)  
sub not permitted  
exceeds height  
floor heights are not in descending order  
wind speed must be > 0  
seismic Ss out of range  
seismic S1 out of range  
seismic TL out of range  
plan dimension L must be > 0  
plan dimension B must be > 0  
heights must be > 0  
weights must be > 0  
Kt out of range (should be between 1 and 3)