bottom chord bracing for cold form trusses

huangyhg

bottom chord bracing for cold form trusses
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

is there a metal deck or a standing seam roof as the roof membrane?
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.

the top chord of the trusses (7 on 12 pitch)will have a rigid insulation board with asphalt shingles, the bottom chord will not have anything (lay-in ceiling below truss)
your question is too vague.  provide more information.  if you do not have a diaphragm, then you have to provide a bracing system in the horizontal plane.
trusses do not brace the top flange of the supporting beam unless the trusses are connected to a rigid diaphragm.   
ba
in your current situation i coincide with abusementpark that since the base of the roofing materials does not provide in-roof plane rigidity, x-bracing should be provided. this x-bracing will warrant lesser displacement of the nodes and so be more for sure usable as bracing points of the beam at the point of contact. however depending upon the hypothesis you consider and the stiffness of the trusses the differences in displacement atop the column might not be as big in all cases as to warrant the need of x-bracing. if there are trusses not falling on columns, but on beams, the difference of lateral displacements should be very apparent.
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.
we did a job recently where architect did not want to use a roof diaphragm.  thus, we had to specify horizontal drag trusses along the bottom chord of the trusses to bring the lateral load fom the vertical lateroal load resisting elements (shear walls in this case).
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.
i apologize for lack of clarity..first time to use this forum.  let me attempt to be more specific.  it is a two story preschool building, structural steel framing.  the second floor is concrete on metal deck, the roof is gabeled pre fabricated metal trusses.  the trusses will be supported by the structural steel beams (all at same elevation) at exterior walls and one interior line of beams (total span about 65').  the truss top chord will have an insulation board, basically foam insulation sandwiched between plywood, whith asphalt shingles.  the bottom chord has no decking, etc.  the vertical lateral load resisting system is moment frames.  if i understand you abusementpark, the truss supplier can design the horizontal bracing between the moment frames given the lateral loads?  thanks for your time!
i would provide the lateral loads to the truss supplier and require him to design and install a horizontal bracing system at the bottom chord level to take the loads to the vertical bracing elements.  you also need to require bracing for the truss bottom chords against uplift (bottom chords in compression due to wind loading).
you were right, i checked with a truss supplier and he said they could design for the diaphragm forces if i give them the shear and drag forces.  i appreciate the information from all of you!