×
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!

*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

diaphragm action question

diaphragm action question

diaphragm action question

(OP)
I am new to Structural field and looking for the principle involved in my following question?

1. As far as I know, Steel deck is supposed to transfer the lateral forces to perimeter walls thru diaphragm action.  I am talking about 22 ga. 1-1/2" deep 3 span cont. deck in a steel frame warehouse type structure.

But some Engineers show horizontal roof bracing along the perimeter in addition to roof deck. What is the purpose of this bracing?

2.  Deck supplier( Canam) catalogue show tables in which deck can be spanned upto 8' c/c, but practically nobody is using joist c/c span > 6' or 6-6" for design of deck( for 22ga. 3-span continuous deck). Why is it so? Why can't I use 8' span for deck?



RE: diaphragm action question

1.  Buildings that you see horizontal bracing in the roof do not have roof deck acting as the diaphragm.  The lateral load is being collected by the bracing which distributes it to the walls and whatever system is being used there for lateral support.

2.  Check the vertical capacity of the deck.  8'-0" is a stretch.  Also check your joist depth.  Spacing joists at 8'-0" o.c. will increase the depth of the joist.  It is a balancing act between structural requirements, economy, and building requirements of the architect, mechanical engineer, electrical engineer, and owner.

RE: diaphragm action question

Good questions.  As far as the local lateral bracing, it's possible they are defining subdiaphragms as required in ASCE 7-02.  Maybe someone else can add to that.  The reason the spans are limited to 6'-0" or 6'-6" has a couple of possibilities.  For one, Factory Mutual only covers 22 gage 1 1/2 deck spanning less than 6'-1".  I've never had that explained to me very well, but I hear it's a good idea not to exceed it.  Also, just because the deck capacity is acceptable for LL + DL doesn't mean that some beefy construction worker carry a tool belt and a roll of roofing material isn't going to exceed the capacity.

RE: diaphragm action question

If bracing is used in the plane of the roof, it may be that the diaphragm is being ignored or perhaps the load or deflection of the diaphragm exceeds the capacity. It seems to be more cost effective to use more fasteners closer to the perimeter. Don't forget that alot of large roof openings take away from the diaphragm's ability to transfer load.

The 6'-0" and 6'-6" limit of the deck span comes from FM (Factory Mutual underwriter) requirements. I understand that some studies were done for a workman on the roof with a fully loaded wheel barrow. This is the maximum deck span at which built up roofing would not tear when subjected to that load.  

I do not recommend exceeding that span limit even if it is a building not underwriten by FM. I believe FM allows you to go 6" beyond those limits to obtain equal joist spacings in a bay. I thought 6'-8" was the limit but it depends upon the deck manufacturer. It is just good practice.

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! Already a Member? Login


Resources

Engineering as It Should Be - Chapter 2: Document Security
This ebook covers basic tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Chapter 2 covers cybersecurity and answers the question: How do you secure your files and documents? Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close