Prepared by: Michael A. Tharp P.E.
Distribution: Customer Proprietary
1.0 Executive Summary and Recommendations
The objective of this structural analysis was to confirm feasibility of the BPI ShadeZone 12’x60’ Octagon Unit withstanding a design wind speed of 110 MPH for an application in Porter Texas.
Both a finite element structural model and structural analysis hand calculations using Roark’s formulas for stress and strain indicate there is no margin of safety for the current design if the design criteria is a constant design wind speed of 110 MPH. Pipe bending stress and foundation bearing pressure are too high.
The difference in wind load pressure for the basic wind speed increasing from 90 MPH to 110 MPH is 20.5 lb/ft2 increasing to 32.3 lb/ft2, or a 57% increase in forces that must be resisted by the structure. The existing structure does not have an adequate margin of safety to withstand a 57% increase in wind load forces.
It is first recommended
the contractor/installer check the local
If a constant 110 MPH is the design criteria the most promising method of modifying the existing structure to withstand this wind speed is to increase the size of the column footings and also install a #5 vertical steel rebar in the center of each 8 inch diameter steel column and fill the columns with concrete prior to installing the rafter corner.
The existing column base bolt size and pattern may not be sufficient for 110 MPH and would need to be checked.
A second method of making the existing structure work would be to install 3/8 inch steel guy wires to the top of the four of the columns to restrain the columns and translate the lateral wind loads into a downward compressive force. This method was quickly ruled out because of the safety issues concerning the use of the structure in a public playground.
A more conventional method of changing the structural design for 110 MPH constant wind speed would involve increasing the size of the vertical columns to 12” diameter steel pipe and also increasing the depth and diameter of the column footers.
A final less conventional solution would be to investigate the possibility of designing a fabric with a tensile strength that actually tears away from the structure at wind speed above 90 MPH or change the general notes in the building plan to indicate the fabric canopy is to be removed if wind speeds greater than 90 MPH are anticipated.