Warped roof plane w/ metal roof deck - Constructability and Diaphragm considerations?

[engjg]

What is the construction feasibility of installing metal roof deck on a warped plane? More specifically 3" deck with 9'9" span, parallel support beams at each ends deck have differing slopes creating a 5°+/- twist or warp.

What are the implications of the warping on the analysis of the a flexible diaphragm?

What are the implications of the warping on metal deck diaphragm design values (if any)?

Anyone aware of any "thou shall not warp your diaphragm" type of code sections in IBC, ASCE 7, AISC 360, AISC 341?

 

[jike]

We, generally, tried to avoid this situation by using tapered insulation but in the case where it could not be avoided we would cut the top of the deck over a support and add a bent cover plate (same gage as the deck) screwed to the deck on each side of the joint. I would think as long as you maintain the minimum area and stiffness of the deck, I do not see a problem with diaphragm capacity.

Obviously, bending the deck in the weak direction is not a problem but it can give an engineer a challenge in trying to slope the structural members. We would sometimes add a tapered member to the top chord of joists/beams as necessary to achieve the required slope.

 

[dik]

like a hyperbolic paraboloid (sp?)? Deep deck is not so flexible You can likely have it rolled to a slight curvature, but, a matter if fitting and holding down until secured to the superstructure beneath.

Dik

 

[dhengr]

Engjg:
Look at the torsional stiffness and the bending stiffness of the sheets of roof decking material, and that will start to determine how much that decking can be twisted and bent to form a warped surface. There are certainly a number of (many) examples of warped surface structures around, saddle shapes, hyperbolic paraboloids, and the like. A sufficiently flexible decking for a highly warped surface, probably means that its support structure must be closer together to support it without allowing buckling, and vica-versa. Then get out your favorite structural analysis software and go at it.

 

[dik]

dhengr... from past experience, other than twisting, the deck is very rigid against bending... unless the amount of flexure is less than 1/2" there will be some significant forces required to 'bend' it.

Dik

 

[KootK]

I've always wondered where the limits lie with this too. Here's what I know:

- I've done tons of large, flat roofs with 1/4" per foot, bi-directional structure sloping (deck warping). It's worked fine and, frankly, I can't imagine being market competitive without making use of that.

- I've done a handful of warped decks with 1/2" per foot, bi-directional structure sloping. Again, no problems that I'm aware of.

- Deck warping is really quite a different phenomenon as compared to straight deck bending. Bending is characterized by longitudinal, flexral stiffness. Warping is really about transverse bending of the plate elements that make up the deck profile. If a deeper deck has similar peak and trough plate widths, I would actually expect a deeper deck to be easier to warp than a shallower deck.

- From a constructability perspective, I think that it comes down to whether or not a worker can depress the deck as required with a reasonable amount of effort. This article gives some guidance for curved deck and comes to the conclusion that 75 lbs of effort is a reasonable bar to set.

- There's no question in my mind that the act of warping the deck induces residual stresses in the deck and, probably more critically, residual stresses in the deck fasteners. I've never considered this explicitly in any of my work and have yet to see evidence of anybody else doing that either. Interestingly, the linked article does suggest consideration of residual stresses in straight deck bending applications.

- Because the nature of deck warping is different than that of deck bending, I suspect that the residual impact on the deck alone (not fasteners) is probably less important than with conventional bending. You'll have transverse flexure simultaneously with your longitudinal flexure. By the book, there will be a Von Mises style stress interaction but I doubt that would be very limiting given that these things tend to fail by way of some manner of buckling long before yield stresses become a problem.

- Back in 2003, I was quite interested in this and created a nifty MathCAD spreadsheet to study the instantaneous slope of a roof when it's warped like this. It's quite interesting. The roof surface will be sloped more like a funnel with things being very steep near the low point and surprising flat near the high point. It makes me question the wisdom of some folks enforcing 1/4" per foot on the diagonal rather than bi-bidirectionally. I'd love to show off my nifty spreadsheet but, unfortunately, it's MathCAD pre-Prime and I don't even have access to the damn thing myself. You can imagine how happy that makes me.







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.

 

[dik]

Thanks KootK... couldn't find my copy of the document... I think for bidirectional warping the thicker the deck, the more difficult it is, even on longer spans.

If you still have it, can you post your mathcad program; I'd be interested in looking at it.

Dik

 

[OldBldgGuy]

I can see twisting a sheet of 3" deck in 10', but the chances of bending it significantly lengthwise in that length are somewhere between slim & none. Your ironworkers aren't going to do it by standing on it, how are they going to pull it down? Some programme might figure out for you exactly how much bend you need longitudinally vs laterally, but I can't really see a solution without a physical test. On the other hand, if you cut the deck at every beam it's easy but those damn architects might not like the less than flowing curves you present.

 

[KootK]

If you still have it, can you post your mathcad program; I'd be interested in looking at it.

I'm afraid that I can't. Since I can't open it myself, there's no way for me to strip it of any sensitive information.

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.

 

[dik]

KootK... thanks

Dik

 

[KootK]

Not sure if it’s still current but I just attempted to send it to your email address.

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.