Column always in Tensions

[ship4885]

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?

 

[Lion06]

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

 

[ship4885]

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.

 

[JAE]

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.

 

[msquared48]

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

 

[apsix]

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

 

[BAretired]

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

BA

 

[JLNJ]

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!

 

[Dinosaur]

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.

 

[graybeach]

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

 

[csd72]

ship4885,

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

Thanks

 

[vandede427]

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.

 

[ship4885]

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.