portal frame gables

huangyhg

portal frame gables
it seems to be a while since single storey portals have come up. every time i design one i wonder about the most efficient way to design the gable frame. if i design it the same as the other portals that is obviously conservative but it does allow the building to be expanded later. then should i try and design a sliding connection at the top of the gable wind posts so they don't take any vertical load and how practical is that connection? if i design the whole gable frame as a simple post and beam construction with bracing in the gable frame, you get a much lighter end frame but given that the manufacturer is making a number of other similar frames i wonder if it really is worth the steel weight saving for another type of frame to be designed, drawn, and fabricated. any thoughts?
carl bauer
i assume you are talking about a typical preengineered steel building.  when a building is meant to expand in the future, the end frame is made like the other portal frames so that the wind columns can be removed.  i don't think that a slip connection is used at the top of each wind column.
daveatkins
both of these solutions are commonly used in the uk.
if a portal is adopted for the gable frame, then generally the connection between the gable posts and the rafter is vertically slotted, so the gable post carries lateral loads only, and the rafter stability checks remain as for the other frames.  wind loads are carried back to the vertical braced bays by a horizontal girder in the plane of the roof.
if a braced frame is employed on the gables then there is a reduction in mass of steelwork required, and a corresponding simplification in fabrication - the connections are simple shear connections rather than moment connections.
i think the question on which is better depends on who you are carrying out the design work for.  if it is for a steelwork contractor on a d & b basis, then they would prefer the minimum weight solution with greatest ease of fabrication.
vb