Bottom chord bracing for cold form trusses
Bottom chord bracing for cold form trusses
(OP)
I have structural steel grid supporting gabled cold form trusses, there is no diaphram at bottom of trusses (top of steel beams). Don't I need to provide xbracing in horizontal plane to transfer lateral roof loads into vertical bracing? Also, do you consider trusses bracing top flange?
thanks
thanks






RE: Bottom chord bracing for cold form trusses
The metal deck can act as a diaphragm, but the standing seam cannot. If there is only a standing seam roof, then you will need to provide x-bracing in the horizontal plane (drag trusses - as we can them) to get the load into the vertical bracing.
I would consider the trusses as bracing to the top flanges of the beams.
RE: Bottom chord bracing for cold form trusses
RE: Bottom chord bracing for cold form trusses
Trusses do not brace the top flange of the supporting beam unless the trusses are connected to a rigid diaphragm.
BA
RE: Bottom chord bracing for cold form trusses
Sound construction warrants the need of at least bracing in the plane of the roof, for without something bracing the inverted V trusses against tumbling you will rely on just cantilever action for their stability and sometimes the connections or roof planes are not good for that kind of stabilization.
Ideally, bracing should be provided at planes intersecting the exterior wall bracing atop the walls; if not feasible, the other chord of the truss may work well, but give a look at the path of the loads where bracing stops, and deformation.
RE: Bottom chord bracing for cold form trusses
These drag trusses were to be designed by the pre-engineered metal roof trusses supplier. We provided a diagram indicating the line loads for which the drag trusses along each exterior wall needed to designed for. The diagram also indicated which walls in the building we were considering to be shear walls. That should be all the information they need.
You do not want to attempt to design the drag trusses since it requires knowledge of the truss bottom chord connectivity conditions, which is not know in the design phase. Just be sure to make it clear on your plans that this is required truss supplier.
Can you elaborate on what the roof membrane is? This "rigid insulation board" has spanning capabilites? We usually call for our pre-engineering cold formed trusses to be spaced at 48" o.c.
RE: Bottom chord bracing for cold form trusses
RE: Bottom chord bracing for cold form trusses
RE: Bottom chord bracing for cold form trusses