×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concrete diaphragm shear strength

Concrete diaphragm shear strength

Concrete diaphragm shear strength

(OP)
I'm almost embarrassed to admit that after a little over four years on the job I've never "designed" a concrete diaphragm.  

I'm working on a large building that has a lot of openings near the braced frames and I want to make sure that we have enough actual diaphragm left at these large openings.

I'm looking in the SDI Diaphragm Design Manual and all of the tables at the back limit the concrete fill to 2 1/2" over the top of the rib.  Why is that?  I haven't read through the entire manual, but I'm not finding anything where I did read.  

Also, why is the fastener layout important for a concrete filled deck?  Is that just for the contribution from the actual deck?

RE: Concrete diaphragm shear strength

StructuralEIT,

Regarding the 2.5" - that is sortof the industry standard for diaphragm tables, not sure of the origination, but generally you have a good bit more than that so the tables are conservative.  The best way I have found to calculate the actual capacity is using Hilti's Profis DF diaphragm design software which, in addition to calculating diaphragm strength using Hilti Pin's of course, will calculate strengths for standard puddle welds etc. with whatever concrete thickness you want.  I verified it against the Diaphragm Design manual and it matches very well:  http://www.us.hilti.com/holus/page/module/home/browse_main.jsf?lang=en&nodeId=-153448

Regarding the fastener layout - yes it just affect the contribution from the actual deck, which can be fairly minimal compared to the concrete itself.

Also if you are in a high seismic area you must check provisions of ACI Chapter 21.11 (in 318-08) for maximum compression forces in the chords etc.   

RE: Concrete diaphragm shear strength

Vulcraft's catalog references the 2 1/2" as the amount of topping in their testing.
 

RE: Concrete diaphragm shear strength

I believe you are dealing with 1.5" decking with a 2.5" fill over, yielding a 4" total thickness slab.  

The side seam and puddle welds limit racking of the individual panels in lateral action, adding to the diaphragm capacity.  

If shear studs are not used, the only mechanism for transferring any composite action between the concrete and steel deck to any drag strut/shear collectors, is the welding of the deck to those members.  

Vulcraft, and similar steel decking manufacturers, have tables of allowable shear for variations of infill, metal gage, rib depth, concrete strength, and fastener type and spacing.  

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

RE: Concrete diaphragm shear strength

I believe you are referring to a composite deck - lightweight or hardrock concrete over metal deck.  Total thickness between 4.5" and 7.25".  The metal deck is usually either 2" or 3" tall.  In the Western US, Verco is the number one supplier and their catalogs contain useful information.

Think of the concrete in a composite deck as the "stiffening component" of the composite deck.  This resists the in-plane shear, similar to how a web resists shear in a wide flange.  However, the concrete does nothing for you with regard to your edges; i.e. transfer shear from your diaphragm to your drag beams ("collectors").

If your diaphragm shear is really high, ignore the table values from the deck manuf and use conventional reinforced concrete calculations to arrive at higher shear capacities (just like a thin concrete shear wall).  For this condition, the deck is just formwork, and you rely on studs for both composite beam behavior and shear transfer.
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources