Diaphragm Tributary

[slickdeals]

Folks,
In designing diaphragms, ASCE 7 requires you to calculate the weight tributary to the diaphragm. Please see attached sketch. I am trying to determine which value is to be used in calculating the diaphragm force.

 

[DaveAtkins]

I would use the reaction based on a propped cantilever.

And I would not assume the base of the wall is fixed.

DaveAtkins

 

[slickdeals]

Is there a specific reason for that? I guess the word "tributary" is kind of misleading.

In Breyer's textbooks, he uses tributary. However, he says that the wall only spans to the roof (He is not assuming a propped cantilever). What the tributary is probably depends on the kind of connection at the diaphragm? Simple spans or continuous across the diaphragm.

In other references, they use the reaction from a propped cantilever.

 

[Grizzman]

TA=24^2/2/20

Unless you're doing some kind of unique design, the wall reinforcing will make the wall act as a cantilevered beam. pinned at bottom and at roof.
Now if you're tying bars into the roof or in some other way making the roof a moment connection, then all bets are off.

 

[Redeemd]

I agree with Dave - use the cantilever reaction if the wall is continuous past the diaphragm. Statics are statics.

If, on the other hand, the wall is discontinuous at the diaphragm (which you might have with interrupted studs which are kicked back down to the diaphragm), then I would use 1/2 story height + parapet.

 

[southard2]

Redeemed and Dave Atkins are right. Also in the newer versions of ASCE 7 the parapet will have a different loading than the wall below the connection point. If the parapet isn't too tall its not going to make much of a difference but it will if you are in a high wind zone and the parapet is tall.

To be honest I've done my analysis both ways. Often I'll use the tributary method just because its faster and usually pretty close. The more off the cuff you are just remember to leave yourself a little bit more factor of safety to cover yourself.



John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[msquared48]

If you run the calcs, the Tw will be 14.4 feet for the propped condition, which I always use.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[asixth]

Tributary areas should only be used for simply supported beams. If you take the tributary area of the cantilever, there is still the fixed-end moment that needs to be accounted for.

I am unfamiliar with the Breyer textbook.

 

[Grizzman]

While his diagram shows fixity at the bottom, both his equations imply pinned at the base. I don't believe his intent was to fix the base. How much good will a <=3' wide footing do in that situation.

 

[msquared48]

I agree with grizzman. Pinned at the base.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask