Framing perpendicular to joists at wall connection

[UcfSE]

re: steel joists, steel deck diaphragm, concrete walls, moderate to high seismic area

Is there a need for framing perpendicular to steel joists, parallel to the deck, for the first few bays between joists at wall connections? Does the deck try to take a compressive force from the wall? I've seen details with and without this, and am having trouble deciding which is right, or if both are right under different conditions.

 

[sdz]

Can you post some pictures?

 

[JAE]

UcfSE - I once looked into this and we did some calculations taking some typical metal deck cross sections and using AISC compression provisions (which is debatible) to see if a gage thickness metal deck with flutes perpendicular to the wall can take typical wind/seismic loads.

We found it generally could, but we still didn't feel comfortable having gage metal laterally bracing heavy walls. Just didn't feel right though.

We have recently used a detail where we lay a C6 over the joists and into the wall (to a clip angle or some other competent connection). The C6 flutes are up. The metal deck then lays over the flutes and you can then weld/screw the deck to the C6 as well as the joists.

Usually we develop the C6 a distance back into the building enough to ensure that the deck-to-C6 connection can take the lateral force. There are a number of variations of C6 spacing along the wall vs. distance into the building but usually the C6's are spaced between 10 to 15 feet and extend into the building 2 to 3 joist spaces.

 

[haynewp]

I remember there has been a little discussion in the past here on this. I have seen it both ways also.

I know you are not allowed to use the deck as the continuous tie when loaded perpendicular to the deck span. For parallel to deck loading, I have never found that you are not allowed to use the deck as the continuous tie (or support reactions from the top of wall) if it can take it. I would create a subdiaphragm using angles connected to the wall then back to the adjacent joists if I was not comfortable with the deck alone taking the load.

FEMA 547 has a little discussion on metal deck bracing walls, I quickly saw something on page 22-16.

http://www.fema.gov/library/viewRecord.do?id=2393

 

[UcfSE]

Thanks for the link haynewp. I'll check it out.

JAE, I tend to agree that it seems the deck should be fine, and I've checked it before a while back. I guess it's striking me as I just don't like it even if the numbers work. The perimeter welds will develop the deck strength also, so I'm not too worried about having the extra framign in order to get load into the diaphragm.

 

[ron9876]

I have also thought about this. If you add framing between the wall and first joist the only thing that you have done is move the condition to the second joist bay. I came to the conclusion that the deck either works or doesn't work in compression.

 

[jike]

On short story, I rely on the deck. On tall story, I add angles. You can also use half a joist space at the outside wall to have more assurity of the deck strength.

 

[ron9876]

But what about the second bay?

 

[haynewp]

You create a subdiaphragm and it is taken out of the deck.

Using a half joist space adjacent to the wall is also a good idea.

 

[ron9876]

Do you mean building a horizontal truss with angles in the first bay? Wouldn't that (depending on the width of the building) be expensive? Do you use the truss top chord as a chord for the horizontal truss?

 

[haynewp]

Yes, it's just like a wood subdiaphragm, if you have Breyer or some other wood text like that it goes through the load paths. It is just not as common in steel deck diaphragms. My observation has been that the perpendicular deck itself has worked adequately over the years for short story, low to moderate seismic and wind areas.

 

[Gumpmaster]

Verco has a detailed calculation using the AISI cold formed code that calculates the combined axial and bending capacity of their deck. If you email their engineering office, they'll send you a copy.

For shorter, thin walls, the deck is likely adequate. When you get into 30ft tall warehouse walls, the outcome may be iffy.

 

[UcfSE]

Thanks for all the great responses!

 

[preynolds]

A similar condition has come up for me as I perform an addition to an existing building. We plan on using common walls of the addition and existing as shear walls, hopefully. When the joists frame parallel to the wall, there is very little information on the top of wall stability or a positive diaphragm connection. The joist closest to the wall is mere inches from the wall. When the joists frame into the wall, there is no top of wall stability detail, either. There are bond beams below joist bearing. How would you handle the situation with the client? If you informed the client about the fact that there may be insufficient stability or diaphragm attachment, but they do not construct it properly, how liable are you as the EOR?

Also, when the joists frame into the wall, the deck is obviously parallel to the wall. However, no angle or structural piece is provided between the wall and deck, therefore, I believe there is no chord element for the diaphragm. Is this correct? I don't quite buy into the deck being the chord element at the boundary. I can buy the force transfer being from the deck, to the chord element, to joist bearing seat (if less than 1600# or so) and into the wall. If the deck did act as the chord, wouldn't you be relying on the last few connections (arbitrary quantity of fasteners) to deliver the boundary force into the joist?

 

[jike]

I have seen many small buildings that have not been designed for a chord element and somehow seem to work OK.

Don't forget that the bond beam, if continuous, can function as a chord or if it is a larger building, the first interior beam or joist could function as the chord. The only trick is how the load gets transfered down to that point and as you said how many fasteners you can count on to make this transfer.

 

[preynolds]

This is a small building. I understand that it works as constructed but I am not sure it works if you tried to calculate a capacity. The problem is, the joists would need to have the axial load indicated on the drawings and that isn't the case. I guess I just like the direct route approach.