Roof bracing for structure without roof diaphragm
Roof bracing for structure without roof diaphragm
(OP)
I am in the process of designing a steel framed industrial building (steel columns, perimeter beams and steel roof joists). Plan dimensions approx. 90' x 90'. 30' bays. The client wants to use a standing rib roof. Cannot count on the standing rib roof to provide any diaphragm action. How can I transfer the forces in the plane of the roof into the vertical frames? A step by step procedure is most appreciated.






RE: Roof bracing for structure without roof diaphragm
Step -1
Ensure all joist connections can transfer tributry shear to supporting beams, connections need to be checked for the resultant shear due to vetical (Gravity) Horizantle (Wind, Seismic).
Step -2
Supporting beams need to be checked for bi-axial flexure due to gravity and lateral loads, these beams will span between columns.
again there connections should capable of transfering both gravity and lateral reactions to columns
Step -3
Since there is no diphragm individual coulmns can take more load depending to there projected area (in case of wind) and supported mass (in case of seismic).
RE: Roof bracing for structure without roof diaphragm
For pre-engineered steel buildings, which also typically have no deck diaphragm but only ribbed metal roofing, they use horizontal in-plane rod X-bracing across at least one bay in each direction. This bracing then transfers the lateral forces along the roof eaves into the sidewall brace lines.
RE: Roof bracing for structure without roof diaphragm
RE: Roof bracing for structure without roof diaphragm
For lateral forces perpendicular to the mainframes, usually, but not always, the X bracing in the roof "diaphragm" coincides with the X bracing or portal frames in the side walls.
For lateral forces parallel to the mainframes at the endwalls, if there is not a mainframe at the endwall, additional X bracing may be needed at the end of the building diaphragm to get the lateral forces down to the ebnd wall X brading. If there is a mainframe, generally, no X bracing in the roof diaphragm is needed.
These buildings are so light that usually wind controls the design, unless Tilt-up panels are present. That situation's a totally different animal.
Mike McCann
MMC Engineering
RE: Roof bracing for structure without roof diaphragm
Another issue which needs to be resolved with steel roofing on steel joists is how the fasteners are done. With purlins, you use screws to fasten the clips for a concealed fastener system, or screws through the crown for exposed fasteners. Does the roofing manufacturer have a method for connecting to joists?
RE: Roof bracing for structure without roof diaphragm
RE: Roof bracing for structure without roof diaphragm
The roof bracing is designed the same way that a truss would be designed, with the larger members and connections towards the end bays.
As the load is transfer to the footing, there will some footings that are required to provide significant hold down capacity.
I detail the roof bracing to lie in the mid-plane of the rafters, however I have seen struts and bracing fixed to the lower flange. This creates additional eccentricity but will help the rafters for lateral-torsional buckling.
Be sure to design the rafters with the additional compression forces because they will behave as the web member in a truss.
And don't forget to design the eaves beam.
RE: Roof bracing for structure without roof diaphragm
An option not mentioned above, which makes all of these issues "go away", is to use a steel roof deck and put the standing seam roof on top of it.
DaveAtkins
RE: Roof bracing for structure without roof diaphragm
RE: Roof bracing for structure without roof diaphragm
RE: Roof bracing for structure without roof diaphragm
I have the impression that your roof is flat. Do you have a standing rib deck that can support your roof loads while supported directly on joists, on a flat roof? I doubt it.
Timing has a lot to do with the outcome of a rain dance.
RE: Roof bracing for structure without roof diaphragm
Can you not use wind portal frames or wind bracing in the walls?
RE: Roof bracing for structure without roof diaphragm
RE: Roof bracing for structure without roof diaphragm
Not many standing seam panels can span very far either if you are in a high wind zone. I used to work for a bar joist manufacturer. Home Depots and Lowes would use standing seam roof panels over bar joists for their garden centers. You might be able to see how they dealt with the lack of a diaphragm off a set of those plans. Keep in mind too that garden centers are open on the sides. I really think you should either go metal building with purlins and cabeling or use a roof deck with the joists.
John Southard, M.S., P.E.
http://www.pdhlibrary.com