Roof bracings attached to deep rafters, how effective? 3

[IJR]

We usually utilize angles or rods to brace roofs in industrial buildings. Usually the rafters are very deep, around 1.5 to 2 ft. There are two techniques in attaching the braces to the rafters, both related to ease of fabrication or erection.

1) Attach the braces to top flanges of rafters or to the web of the rafters

2) Attach them to purlins with a gusset plate attached to purlins.

I keep convincing myself the deep rafters will not really benefit from the bracing action given their depth and this method of connection. Am I being theoretical?

Regards
IJR

 

[JAE]

IJR are you talking about horizontal plane bracing??? for lateral roof diaphragm action?

 

[IJR]

Thanks for fast response

Exact JAE, it is the horizontal bracing for lateral roof diaphragm action, usually aimed for wind load transfer to vertical bracings in the plane of walls in longitudinal direction of a building.

Similar system for diaphragm action of floors in industrial buildings


While here let me emphasize my question. Deep beam or rafter and braces attached to the web by a gusset plate parallel to the flange of the beam or bolted directly to top or bottom flange of the beam. How effective?

I will appreciate your comments.
IJR

 

[Ron]

IJR...
ANY bracing will help, but you can choose your load path on which to concentrate...

If your diaphragm consideration is high, then an upper flange load path would be more beneficial to strengthen. This will allow stiffening of the diaphragm and perhaps even reduce the torsional effect on the rafters, as long as the strengthening is continuous.

The torsional effect on the rafters has to be checked. If Iy/Ix were 1.0, it would not be an issue (assuming adequate design for loading!); however, since this is not likely and the Iy/Ix value might be low, then you might consider a combination brace to accomodate both the diaphragm stiffening as well as the torsional or multi-plane bending of the rafters. X-bracing or web-attached bracing should work for these. Be careful not to use discontinuous or offset X-bracing as this can do funny things to your diaphragm distribution.

 

[JAE]

I wouldn't attach to small roof purlins as this requires an additional load path through the purlin connection into the supporting roof girders. As Ron stated, attaching to the top flange, or to the web near the top flange, is what you normally see in metal buildings.

 

[dik]

Can you use the roof deck and transfer the loads to the perimeter beams and attach the deck as required. The attach the perimeter beams to the x-bracing?

Everything at the high level... or did I miss the question?

 

[IJR]

First thanks to all of you fellow great guys.

Dik : Our roof deck are often too flexible, or we simply dont want to trust the guys who fix it. No diaphragm action assumed. Instead to transmit wind load to side bracings, we would brace the roof.

If you comment on that, would you please comment also on what you were trying to say on use of roof deck and perimeter beams and x-bracing combination. I probably missed you there.

Regards
IJR

 

[dik]

About 90% of the steel buildings I've designed use the roof deck as a diaphragm to transfer the lateral loads.

Roof deck is typically 1-1/2"-22Ga mat'l (I don't know what percentage of the people in the forum use SI, so will use Imperial) and joist spans vary from 6' to 7'. I have several packages of information on the use of deck as a diaphragm, and many of the deck suppliers used to provide tables for allowable shear/defl and fastening using 'puddle' welds to connect the deck to the supporting superstructure. I'm pretty sure one of the design manuals came from Westeel-Rosco. A single angle is welded to the top of the joist at the seat location and the deck is secured to this; the area of steel for the angle is determined by the moment caused by lateral loading. TOS for edge beams parallel to joists is at the same elevation.

Because of the large areas involved, the shear values are relatively small and it's not normally a problem; I've not used this in areas where seismic stuff governs.

With respect to angle bracing, I've usually braced at the US of the joist structure, carrying it through to the exterior walls. There seems to be less stuff in the way, down there.

 

[IJR]

Dik

That application seems interesting to me. I have actually tried to put diaphragm action into use, with no luck, because no data is available from manufacturers in my area. The only data provided is usually load capacity tables.

As I mentioned before, in my area guys who do cladding are a different team from the guys who design the framework. So there is a habit not to trust their application.

However, I am interested in the subject. Is your detail discussed in MBMA 1996 manual, because I am planning to buy it.

If you come across a web site, please let me know.

Regs

IJR