V Shaped Steel Joist?

[SteelPE]

I have a project where the client was adamant about having a column free space. So the project utilizes 80' long span steel joists. Due to the architectural design of the building and the type of construction the roof joists were set flat with tapered insulation being utilized to provide roof slope to the internal drainage system.

The project design was completed 6 months ago and went through 2 rounds of bidding. The contractor that was awarded the project wants to eliminate the need of tapered insulation and instead utilize the steel to achieve the necessary roof pitch. There only two ways I can see them doing this is to provide a double pitched top chord joist. One with a gable pitch and another with a V shaped top chord. During the design process the gable pitched joist system was eliminated due to architectural requirements leaving the proposed system to be a V shaped top chord.

Working for a steel fabricator for 13 years (and 5 years out on my own) I have never seen this type of joist system before. Is this just my inexperience, or is this just a really odd type of roof the contractor is proposing?

 

[dik]

I was involved with a project where the Architect wanted a 'space frame' appearance and used joists sloped at a 45deg angle. HSS web members were flattened at the ends and welded to the 'fillet' part of the single angle chords. Worked well.

Dik

 

[ajh1]

This feels like a very stupid approach. They are creating a ponding situation right at the worst possible location for such to occur. The internal drain lines will need to run from the center of the building back to the walls since it is a column free space. Any deflection created by load on the joists will magnify the potential ponding assuming (as can be likely expected) that the drain will plug at some point in time. If the ponding grows, then the deflection grows, etc. etc.

 

[dhengr]

SteelPE:
I’d want to use the gable arrangement of trusses/OWSJ’s. This gives you the max. joist depth where you need it, about (40/8 = 5" extra depth) slope, maybe 8 or 10" of slope for roof drainage. What Arch. problems?..., for goodness sakes, add 10-12" of height to the parapet around the bldg. This drains to the outside (two ext. sides) of the bldg. where the piping is a few foot horiz. run from the roof drains and then down the ext. wall. Put a couple feet of sloped insul. from the parapet back into the bldg. to get drains a few feet away from all the ext. wall/joist brg./parapet flashing details, etc. Add scuppers for secondary drainage, if needed.

The stl. joist manuf’er. can do either, at some extra cost, but the “V” shape drains to the center, where drain piping will have to run horiz., serving a number of drains in each direction to the ext. walls at the ends of the bldg. If and drains plug up, you have one hell of a ponding problem, and no easy means of secondary drainage. Doesn’t the code now require some secondary drainage means for plugged primary roof drains?

 

[SteelPE]

Architecturally the gables don't work for 2 reasons of which I won't get into

Let me just say that I don't like the idea either. Problem is the contractor has the owners ear and now the genie is out of the bottle and I have to tell them why it's not a good idea. But with $ on the line I am not sure I am going to be able to sell how bad the idea is to the owner. I am sure the GC is going to sell it to the owner using the secondary drainage system against me saying "well, we are installing a secondary drainage system so what's the problem?".

I suppose I will just tell them if they want to approach the problem this way then adding a line of steel in the building is a required.

 

[jike]

We have designed many similar buildings with the gable configuration for the joists. I know of no architectural problems that cannot be overcome with a little creativity. The significant ponding issue and secondary roof drains are major considerations that should drive the design. I even question whether a truss with top chord angle sloping down to the center is even structurally feasible. What happens to the top chord vertical component force which pushes down at the center? I know of no truss that has ever been built this way.

Contractor is most likely looking at the fact that he will have only one line of roof drains.

I am in total agreement with ajh1 and dhengr. Don't let the contractor, architect and owner push you into a poor structural design. There is more to a design than $.

 

[KootK]

Somewhat dissenting opinion here.

1) Is one of these V-trusses an oddity? Heck yeah.

2) Is one of these V-trusses structurally viable? Of course. It's just a bunch of triangles, like any other truss. If you hold your center of truss depth, as I expect, your V-truss will actually be stiffer than your flat trusses. Less deflection.

3) Is one of these V-trusses practical to fabricate? Dunno. Give your local OWSJ guy a call and find out. I doubt it's a big deal with modern fab techniques. Worst case scenario, splice together two mono-slope trusses and accept whatever premium comes with that. We know that's possible.

4) Is the V-truss business a worse ponding system than the gable truss? Yep. However, at the end of the day, it's just another design issue to be solved. Do the requisite ponding checks, be conservative in that, and let the design team know that the results embody a premium for doing it this way.

5) Is the V-truss business a worse ponding system than what you have now. No, at least not if I understand your situation correctly. I believe that you currently have a roof plane slopped to interior drainage at the center of a very long truss. I'd be careful with this. If you cry foul because of drainage, and somebody clues in that the system that's been out to bid twice already has that same issue, you may find yourself in an uncomfortable position to defend.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I wouldn't have a problem provided the contractor provides calculations and shop drawings sealed by the joist manufacture/provider's engineer. If you are curious and want to know if it is practical, contact a major manufacture (Vulcraft or . . .). Otherwise, let the contractor do the leg work. I am surprised the contractor thinks it will be cheaper than tapered insulation.

 

[hokie66]

Internal drains, yukk. That's my only comment.