Ridge Vents in a Steel Diaphragm Roof

[vincentpa]

I am designing a rectangular CMU building (42'x60') with single span cold formed steel trusses with a 4 on 12 roof pitch. I am using 1.5 Type B roof deck spanning perpendicular to the trusses as a diaphragm attached with #12 screws. Metal roofing will be placed on top of the metal roof deck. The architect wants to put a continuous ridge vent at the ridge the entire length. I believe that this will interupt my diaphragm. Does anyone have any ideas on how to keep continuity in my diaphragm?

 

[DaveAtkins]

This comes up in wood roof diaphragms a lot. My answer is to not worry about it. The ridge vent, or slot, in the diaphragm, occurs at midspan of the diaphragm, where the shear is zero. The moment in the diaphragm is maximum at midspan, but this is resisted by the chords, not the steel deck.

DaveAtkins

 

[jike]

Consider them as 2 diaphragms. The 2 diaphragms should be stable if they can be resisted by the walls on 3 sides of each diaphragm. The two side walls need to be able to resist the torsion.

 

[JAE]

Dave, I would generally agree with you not to worry about the slot in the diaphragm, but the long slot does affect lateral loading perpendicular to the slot in that instead of an "X" ft. deep diaphragm you now have two separate X/2 ft. deep diaphragms.

This would greatly affect diaphragm shear stiffness and possibly affect the design in that the flexible/rigid classification for diaphragms depends on relative diaphragm flexibility. You also may have chord force issues as well.

But in most "small" building cases, it may not be all that critical as you suggest. Just thought I'd point out my thoughts here.

 

[haynewp]

I think blocking is normally used at each side of the ridge vent in wood construction. I have seen a lot of disagreement between engineers as to whether to ignore the vent or not, and just that in itself makes me believe that it is not usually an issue and distress in the vent area is not frequently noticed.

But, if it acts as 2 separate diaphragms when load is perpendicular to the ridge it, seems that something should be done along the boudaries at each side of the ridge. I would install blocking between the metal stud trusses at each side. Check the chord forces if it actually were acting as 2 separate diaphragms with load perpendicular to the ridge and see what you get.

 

[vincentpa]

I thought of the the diaprhagms as two separate diaphragms myself. But how do you create a chord at each end of the ridge with Cold Formed Steel Framing (CFSF)? This is a good method if it can be done with CFSF.

Another way of handling it (I read in the APA "Introduction for Lateral Design" for wood diaphragms) is to block a portion of the vent and double the attachements in the blocked portion of the ridge. The ridge vent would not be continuous and the diaphragm would be treated as having holes in it. This sounds applicable to my situation. Another question arises, how do you "block" a CFS Truss with light gage steel to create a tie for a steel deck diaphragm? Are there CFSF blocking members?

I would like to ignore the vent as Dave Atkins says. He is probably correct with such a small building, but I just don't feel right about it. It is just the perfectionist in me.

 

[haynewp]

The manufacturer's truss shapes I have had delivered on my projects were not easy to install blocking between the top chords. You can see here that some top chord shapes are only a couple of inches deep and it was difficult to get an attachment of a stud into the side of one them.

http://www.icc-es.org/reports/pdf_files/UBC/5868.pdf

You can run a continuous hat channel attached underneath with a couple of screws at each truss. Or somebody else here may have a better idea.

 

[civilperson]

One idea: Tie the two halves of the metal decking together with a series of steel angle or strap braces, say 12" spacing with alternate braces at 45 degrees to joists.

 

[DaveAtkins]

OK--I was thinking of wind PARALLEL to the ridge vent (obviously less of a problem).
In the past I have also looked at a wood diaphragm with a ridge vent, with wind PERPENDICULAR to the vent. What happens is, near each end of the diaphragm, where the shear is largest, there is horizontal shear (parallel to the ridge vent) from the roof sheathing on each side of the vent. These equal and opposite shears must be resisted by weak axis bending of the top chords of the trusses.

DaveAtkins

 

[jike]

With two half diaphragms and wind perpendicular to the slot, the load on one diaphragm is from direct positive wind pressure on the windward wall plus the windward half of the roof and the load on the other diaphragm is from wind suction on the leeward wall plus the leeward half of the roof.

 

[AggieYank]

I'd worry less about the wind load perpendicular to the slot in the diaphragm, assuming you have some kind of chord next to the opening.

I'd worry more about the wind load parallel to the slot. In theory, you've broken the diaphragm into two separate diaphragms. Each diaphragm will be unsupported on the slot edge. The only way this works is if the chords at the edge of the building (perpendicular to the slot) can make the diaphragm continuous, or if the building can resist the torsion that will result from only one side being braced (and both sides in the other direction).

 

[vincentpa]

I have been thinking about the problem more and more. When the wind is perpendicular to the ridge, I have two options.
1. Block every other space between the trusses and double the connections as if there are holes in the diaphragm. The Arch might not like this. The ridge vent would not be continuous.
2. Provide continuous blocking at either side of a the vent. Then I would put continuous straps on top of the blocking to act as a chord. This would make the diaphragm two diaphragms.

When the wind is parallel to the ridge is a bit more difficult.
1. I can do as I would in item one when the wind is perpendicular to the roof but the ridge vent would not be continuous.

2. I can do what I would do in item two above. Then place light gage straps between the two sets of blocking on either side of the vent. This would make the two diaphragms in the perpendicular direction one diaphragm in the parallel direction.

3. I can run a ridge drag beam under the top chord peak and use light gage straps to drag the wind into the shear walls.

I am a little worried on how I will get the forces into the chords (cmu bond beams) when the wind is parallel to the ridge. The end trusses sit on top of the CMU wall. The peak on the truss is 7' tall. I am having a hard time visualizing this. I usually deal with flat roof diaphragms.

 

[twinnell]

Try contacting a steel truss manufacturer such as TrusSteel. Or even The Steel Network and ask for their opinion. This is stuff they see on a daily basis.

 

[vincentpa]

I have and they want me to pay them to tell me how to do it. I have a problem paying these guys for some advice if I specify their product. I wonder who is running the show there. I REFUSE to specify a TrusSteel product for this reason. Customer service is key. I hope they fold!!!

 

[twinnell]

I have never worked with them. That is dissapointing. You would expect them to help you out as much as possible...without running calcs...so that you spec their product. I will keep that in mind when I spec another steel truss.

 

[jike]

It seems to me that adding blocking with strapping between trusses is alot of work. How about a continuous light gage angle with the long leg laying flat against the truss top chord and screwed to it. The short leg (1 1/2" max.) could project upward and get incorporated into blocking along the edge of the vent. If this doesn't work architecturally, the short leg could perhaps be turned downward and coped at each truss.

Just an idea....

 

[DaveAtkins]

As jike mentioned above, for wind parallel to the ridge vent, can't you look at this as two separate three-sided diaphragms?

DaveAtkins

 

[vincentpa]

I have information for the design of a three sided truss for a flat diaphragm. I am a little confused or unable to visualize on how the chord force works in a gable end truss if the wind is perpendicular to the ridge for a three sided condition. To tell you the truth, I am unable to visualized how a the gable truss works as a chord without tipping over or bending. Can anyone explain?

 

[jike]

Perhaps I misunderstood your question; do you mean when the wind is parallel to the ridge? In the parallel wind condition, the gable end trusses take out the shear developed from the torsion produced by the eccentricity of the applied loads versus the resisting loads on the 3 sided diaphragm. Additionally, the joint between the top of the wall and the bottom chord of the gable end trusses needs to be braced against direct wind pressure or wind suction.

I hope this helps!

 

[vincentpa]

The problem is wind parallel and perpendicular.