2015-6-25-bowstring-truss-part-2

Bowstring Truss Failures, Part 2: Repair Methods

DonanEngineering Commercial & Residential Roofing, Forensic Engineering, Structural Damage & Collapse

Have you checked out Bowstring Truss Failures, Part 1: Deficiencies & Failure Causes?

 

Typical Bowstring Truss Repair Methods

Bowstring trusses can be retrofitted to remedy design problems and to increase the load capacity to meet the current code requirements. This is accomplished by first analyzing the trusses using frame analysis software with the current code-required loading and then upgrading each member and connection as required to meet the design requirements. The bowstring trusses can be repaired using posttensioned rods for the chord members and steel strap reinforcement for the connections.

Tension Reinforcement of the Bottom Chord

Tension Reinforcement of the Bottom Chord

Web Member Connection Reinforcement

Web Member Connection Reinforcement

Original truss design calculations are rarely available for review when reinforcement of trusses is performed. As such, it is imperative that reinforcement includes both bottom chord tension rods and straps at connections of web members. Many times owners will choose to add posttension rods as reinforcement because the bottom chord is readily accessible without any significant demolition or other modification of building components. While tension reinforcement will increase the tensile capacity of the bottom chord, if the tension (bottom) chord is not the controlling member for the truss design, the bottom chord reinforcement will not increase the overall capacity of the truss. It is not uncommon to see failure of the unreinforced web member connections and top chord of the truss in a truss with bottom chord reinforcement.

Conclusion

Bowstring trusses are an efficient and effective method of providing large unobstructed spans in warehouse and industrial buildings. Design deficiencies, along with reduced allowable stresses, have now determined that bowstring trusses can be significantly prone to failure. Taking a proactive approach in reinforcing bowstring trusses before failure, in addition to making the building safer, is also more cost effective than remediating a collapse.