Axial force in joists and joist girders from wind load 1

[Recent Grad Josh]

Hello,

I'm working on a 420'x120' warehouse building made of precast concrete walls. There is one row of columns in the middle of the building, and the metal roof deck is supported by joists at 5' ctrs. The joists span 60' between the wall and the joist girders (which span between the columns).

I calculated the axial force in the joists and the joist girders using this: (wind pressure)*(wall height)/2*(tributary width) where the tributary width was 5' for the joists and 60' for the joist girder. The axial force this gives seems very high to me and I am wondering if I am looking at this incorrectly. Any help would be appreciated! Thanks!

 

[KootK]

The joist sound right. Normally one would assume no wind load to the girders though. Where your deck flutes are normal to the walls, it's typically assumed that the load goes straight into the deck. Of course, that results in axial deck load to be considered. Some folks to, some folks don't. Where it causes trouble, you can do spiffy things like putting HSS in the flutes of the first bay or two or installing a horizontal truss below the deck.

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.

 

[Deker]

That is essentially correct, but presumably you have a parapet with a higher wind pressure that also needs to be accounted for. I recommend reading this document for a good discussion of load path in these types of structures: Link. Although it is written for a hybrid panelized roof, the same principles can be applied to your building.

 

[Recent Grad Josh]

Thanks a lot, guys!

 

[dik]

Deker: good link...

Dik

 

[DaveAtkins]

I may be missing something, but I don't agree with how you are looking at this. The wind load from the top of the PCC wall should go directly into the steel deck diaphragm, and should not cause axial force in the joist top chords.

DaveAtkins

 

[sticksandtriangles]

I will second Dave on this.

 

[KootK]

I'll second Dave on it where the deck runs perpendicular to the walls. That's essentially as I described it above. Where the deck runs parallel to the walls, however the load definitely spends some time in the joist top chord unless unusual measures are taken to prevent that.

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.

 

[jayrod12]

I think this is one of those things that most people ignore, i.e. the axial force that spends some time in the joist while the joist transfers it to the steel deck.

 

[KootK]

Unless its a basement lid etc the compression force in the top chord of the joists is usually pretty small compared to the flexural compression that develops.

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.

 

[McSEpllc]

Page 4 of this article addresses that:

https://www.aisc.org/globalassets/modern-steel/archives/1996/03/1996v03_joists.pdf

Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[DaveAtkins]

We are touching on one of those questions I have had (and it appears others have had as well!) over the years. For the most part, I agree with KootK and jayrod12. The joist top chords do see some axial force. However, contrary to that AISC article, I would NOT put these forces on the drawings for the joist supplier--they are negligible.

In the past, I HAVE put axial forces and end moments on the drawings for the joist supplier--when the joist is part of a rigid frame or is a drag strut (collector) carrying a significant axial force.

Now, I also think the steel deck, when it spans parallel to the wall, CAN take diaphragm force. We tend to think, "How can a deck take axial force perpendicular to the deck span? The deck is flimsy in that direction and will crumple." But it is a SYSTEM, and the joists and deck work together to receive and transfer diaphragm forces.

DaveAtkins

 

[KootK]

We tend to think, "How can a deck take axial force perpendicular to the deck span? The deck is flimsy in that direction and will crumple."

I'm solidly in that camp.

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.

 

[jayrod12]

I'm solidly in that camp.

Can you then explain the reasoning behind the deck suppliers not indicating a lower diaphragm strength or stiffness depending on direction of deck?

 

[JAE]

Can you then explain the reasoning behind the deck suppliers not indicating a lower diaphragm strength or stiffness depending on direction of deck?

You're talking about two different things.

There is a difference between:
1) Compression forces applied to a deck in its plane (how the force gets into the diaphragm)
vs.
2) Shear capacity and stiffness for overall diaphragm distortion (how the diaphragm resists the shear in-plane where shear capacity and stiffness are non-directional).




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[KootK]

Just what JAE said.

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.

 

[jayrod12]

So do you two provide framing in the first joist space to drive the load into the diaphragm? Akin to wood blocking.

I envision the whole accordion type failure, but if I specified angles at 6 ft o/c in the first joist space I would get crucified.

 

[DaveAtkins]

I typically provide an edge angle perpendicular to the joists, sitting on top of the joists and connected to the wall, with the deck edge fastened to this angle. Then I assume the lateral load on the wall goes directly into the deck--not through the joists. In reality, I think it goes into both deck and joists.

DaveAtkins

 

[KootK]

So do you two provide framing in the first joist space to drive the load into the diaphragm? Akin to wood blocking

You're talking about the case where the joists run parallel to the wall, right? If so then, like you, I normally would let the deck take the load in compression unless the loads were just too high for that. If the loads were too high for that path then, yes, I'd throw some hss in the flutes for the first joist spacing or two. And it would be unpopular. No accordion action either way though.

I typically provide an edge angle perpendicular to the joists, sitting on top of the joists and connected to the wall, with the deck edge fastened to this angle

Me too but I'll assume that the angle, or the top of the wall itself, is spanning out to the joists with the deck giving way accordion style.

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.

 

[haynewp]

I worked at a company before that designed lots of large retail box buildings and many were in high wind areas, some of which have experienced hurricanes. There were never any axial loads indicated on the joists to brace the typical wall case that I can recall. Not that there isn't some interaction between the stiffness of the joists with the stiffness of the deck and attachments, but I think this is one of those things that has been ignored for many years and tends to work. We don't, however, use the weak direction of the deck for bracing floor girders during concrete pours, which seems like somewhat of a contradiction.