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- excerpt of study regarding structural analysis
excerpt of study regarding structural analysis
Posted by Morgan White on February 16, 2015
The existing structure was assessed based on the original 1966 drawings prepared by Lev Zetlin and Associates (see Appendix A). The current roof structure consists of a 6 in. reinforced concrete slab supported by steel trusses that span 84 ft across the Great Hall. The trusses are made up of teeshaped chord members and double-angle web members. Secondary wide flange beams spaced at approximately 21 ft span between the trusses and create two-way action in the slab. Analyses conducted during Phase 1 of this Feasibility Study showed that the secondary components (roof slab and beams) do not have sufficient capacity to support the added load of the photovoltaic array. Consequently, support systems were conceptualized that would deliver the new loads directly to the trusses. Analyses conducted during Phase 2 therefore focused on the roof trusses.
Loads and stresses on the trusses were determined using the American Society of Civil Engineers (ASCE) 7-10 design loads and typical material weights. Taking into consideration the dead loads (truss members, roofing materials, concrete slab, catwalks, and ceilings) and snow loads, the total load on the roof structure is estimated to be 130 lbs per sq. ft (see table below). With a distance of 21 ft-4 in. between trusses, this equates to approximately 2,800 lbs per lineal foot applied to the truss.
- Existing Load Summary - Load (lb/ft2)
- Truss (Self) 10
- Slab 75
- Catwalks and Ceilings 13
- EPDM Roof 5
- Insulation 3
- Live Loads (Snow) 25
- Total 131 lb/ft2
Using structural analysis software and hand calculations, it was determined that the members of the existing roof trusses labeled T-3 in the original drawings do not meet the current structural steel code for compression capacity under these load conditions. At the time of original construction, the members met requirements called for by the applicable code, the American Institute of Steel Construction (AISC) 6th Edition Steel Construction Manual. The AISC Steel Construction Manual has since been updated to account for torsional and flexural-torsional buckling and strength limitations of slender elements within members.
The trusses were also analyzed for the addition of the PV cells and accompanying concrete knee walls. The weights of these materials and associated snow drifting loads would apply an additional 30 lbs per sq. ft (see table below), or approximately 665 lbs per lin. ft to the T-3 trusses at the locations of the new walls.
- Additional Load Summary - Load (lb/ft2)
- Concrete Wall 18.75
- Photovoltaic Cells 5
- Snow Drift 7
- Total 31 lb/ft2
These loads would introduce an additional sixty thousand pounds (approximate) into each of the steel chord members, causing four additional chord members to be stressed beyond code-prescribed limitations