×
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

Column always in Tensions

Column always in Tensions

Column always in Tensions

(OP)
The building I am workign on has a significant portion of the building cantilevered out. We have created 3 large trusses that carry this cantilever slab, that look like giant P's. Since the cantilevered portion of the floor is longer than the backspan, we have significant uplift on the first column of each of the trusses. There is ALWAYS a tension on these columns. Are there any special code provisions to consider with a column that is always in tension?

RE: Column always in Tensions

Not that I'm aware of, but I'd be careful when considering lateral loads.  What type of foundation system are you using?

RE: Column always in Tensions

(OP)
The tension columns sit on top of a very large foundation wall supported by a large strip footing.

Only one of these trusses has an X down to the base of the building, the lateral system is completely seperate from the trusses, though analysis shows their stiffness does attract some lateral load - the lateral system has been designed as if these trusses do not exist.

RE: Column always in Tensions

Along with StructuralEIT I'm not aware of any special code provisions for tension columns.

But I would suggest the following:  With a tension element (be it a column or a hanger) you have a very non-redundant condition.  If the column fails - boom!

Thus, as a prudent engineer I'd consider an additional level of safety factor, perhaps 1.5 to 2 in addition to your code required levels of safety.

 

RE: Column always in Tensions

Yes, there are, but mainly in restrictions in truss tension members (rods or angles) to resist slapping action that would affect drywall finishes for enclosed members.  Not really applicable here though due to the shape of the member.

Mike McCann
MMC Engineering

RE: Column always in Tensions

As above, don't be tempted to go too small, serviceabilty will suffer.

RE: Column always in Tensions

As stated by others, make sure that the column is properly tied down.

BA

RE: Column always in Tensions

Carefully go through the shop drawings.

There may be details which are non-critical no-brainers under compression (such as base plate and anchor bolt details) which are of critical importance under tensile loading. A detailer may or may not know the difference.

Another example is the column splice. Column splices may usually be detailed for bearing but under tension there is no bearing!
  

RE: Column always in Tensions

Look in the geotechnical requirements.  There are often special factors of safety for things like this.

When you get done, add some extra tension rods to the design above the number required by the calculations.  It is cheap insurance.  Say they are for future maintenance or something.

RE: Column always in Tensions

Make sure creep deflections won't be significant.  Consider post-tensioning the columns.

RE: Column always in Tensions

ship4885,

Are these steel or concrete columns?
Also can you clarify what the foundation are going to look like.

Thanks

RE: Column always in Tensions

I would spend a lot of time detailing the connection of the tension column to the foundation wall. Instead of using your typical anchor bolts, I would probably use long threaded rods as "dowels" cast into the walls with a nut at the bottom.

The last thing you want is for some J-bolt to straighten out on you and pull out of the hole.

RE: Column always in Tensions

(OP)
The foundation system for the building is spread footings. ALl of the tension columns sit on a 4'0" thick concrete wall on top of a 15ft wide, 3'-0" deep footing. The wall is approx 36 ft tall.

The columns are steel, and are MORE than appropriately sized for the tensions forced, stressed to about 10% or less, but sized so high to help control deflections.

Though not req'd by the code we are using, we have done a collapse analysis of removing one of 9 different columns, 3 of which are the tensions columns, and no strength failures occur, and no drastic deflections occur. The building uses 2 "distribution trusses" in the direction transverse to the 3 main load carrying trusses, which will send load back and forth to the other trusses if one is damaged or has a lesser stiffness.

I think as most of you stated, the detailing of connections will be most critical in this project.  

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