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Extending Existing Metal Deck

Extending Existing Metal Deck

Extending Existing Metal Deck

(OP)
Lets say you have an exterior braced frame. You want to move the braced frame outward 2'. Therefore you need to extend the roof deck another 2'. Is it feasible to just weld a 2' deck extension on to the existing beam and you new beam which is 2' farther out? See attached:
https://www.dropbox.com/s/w4ijgncg214cu6e/Deck%20s...

I don't like the idea of this due to the eccentric shear. However they have to lap deck panels at some point (they are not infinitely long). Is there a certain lap you could try and achieve? Is there a good reference to analyze this situation so that the deck and welds can be designed for the eccentricity?

I'm inclined to go with a horizontal truss (there would also be a vertical truss in the attached case). In this case there is about 20 kips of force which turns out to be about 200plf.

Thanks!

EIT
www.HowToEngineer.com

RE: Extending Existing Metal Deck

I would suggest cutting back existing deck to first interior support and lap and weld new deck at that point.

RE: Extending Existing Metal Deck

It's important to recognize that, per standard diaphragm design assumptions, the shear eccentricity doesn't actually affect your deck. It only affects the chords and struts that you will need to supply around all four sides of the new shear panel that you will be creating. One of your struts will be your deck support angle at the new beam line. I'd need to see a plan layout to identify the other three boundary elements.

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.

RE: Extending Existing Metal Deck

(OP)
Jike -> for some reason I also feel more comfortable taking it back to the first joist.

KootK, Are you saying that the eccentricity would theoretically be taken out in the top and bottom chords?

I mean if we stick with "beam theory" then this would be like and extended shear tab or something to this effect. However I could put in chord extensions at each end. Plan is rectangular with bracing on all 4 sides.

Are there issues with lapping the new deck on top of the old?

EIT
www.HowToEngineer.com

RE: Extending Existing Metal Deck

Quote (OP)

KootK, Are you saying that the eccentricity would theoretically be taken out in the top and bottom chords?

Not quite. See the sketch below. It may not be earth shattering. My point is merely that the only effect on the deck will be a slight increase in shear. The deck won't be subject to in-plane bending stresses of any sort. At least, this is what would happen when one applies the usual "shear panel" design assumptions.

Any in-plane moment in the diaphragm due to eccentricity would wind up being resisted by axial force in the chord members at the boundaries of the diaphragm. Consequently, the deck "splice" would only need to transmit shear, just like any other deck panel to deck panel connection.

I drew the deck splice between framing members in my sketch below. I did it that way only because I thought it more illustrative and because you instructed us to ignore vertical load issues. Know that I absolutely agree with jike: execute your lap above a framing member. I don't even think that you have a choice about this. Among other things, one of your deck capacity checks is shear buckling in the deck. I'm not even sure how one would go about lapping deck at mid-span such that the assumed shear buckling capacity would be restored.

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.

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