Roof bracing for structure without roof diaphragm

[Ipetu]

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.

 

[StructGen]

You have to relay on your connections over all,
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).

 

[JAE]

I'm not sure you can take a three-bay steel framed structure and rely on weak axis stiffness of joists and beams to resist lateral flex in the roof diaphragm. There's just not enough stiffness there.

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.

 

[hokie66]

JAE is correct. You have to design a horizontal truss in the roof plane to take the lateral forces to the edges. In portal frame buildings, this is typically done in the other direction by struts and cross bracing just below the rafter top flange. But these buildings use cold formed purlins, not bar joists. I would think with bar joists, you would have to fashion some sort of truss below the joists, with the joists used to prevent the truss from sagging in its weak direction. Steel roofing is not really compatible with bar joists.

 

[msquared48]

More points to consider here -

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

 

[hokie66]

Ipetu,

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?

 

[JAE]

I wonder if the building is a pre-engineered structure since Ipetu didn't mention that - I was just using it to make my point.

 

[asixth]

We always try to provide horizontal roof bracing, you could detail moment-frame action in the strong axis of the building but this may lead to over sized columns and excessive drift during 20yr wind events.

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.

 

[DaveAtkins]

And don't forget the joists must be specially designed by the manufacturer for gravity loads as well, since joist tables assume the top chord is braced.

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

 

[hokie66]

That would bear looking into, Dave. You might have to use battens between the two layers to allow for alignment of the standing seam clips.

 

[Ipetu]

Thanks very much to all that posted answers. JAE, the building is not pre-engineered / pre-manufactured. DaveAtkins, I have recommended to the client to use a steel roof deck with the standing seam roof on top of the steel roof deck. However, the client has so far rejected that option.

 

[paddingtongreen]

I'm a little confused here. Standing rib roofing is usually placed over a solid substrate, on a sloped surface, at least 3" in 12". The Architects pick it for its appearance.

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.

 

[geoffwatson]

If the trusses are engineer designed you could portalise the frame with the trusses across the building and if you have edge trusses ditto along the building but you would need heavy sections on the weak axis and the connections can be tricky.
Can you not use wind portal frames or wind bracing in the walls?

 

[Ipetu]

I have included wind bracing in each of the four walls.

 

[southard2]

I realize this thread is a bit dead. One also needs to consider that standing seam roof panels directly over bar joist won't brace the top chords of the joist laterally. Bar joist manufacture design the joist with closely spaced X bridging. This over a 90ft x 90 ft building will be expensive. I suggest using a steel roof deck diaphragm. Simply screw the standing seam panels down to the roof deck with dekfast or concealor concealed screws.

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