Pre-engineered building - Tie Rods

[Stazz]

I just wanted to get some input in how you guys tweek the Baseplate - Tie Rod interaction on these buildings:

I have 135' clear span pre-engineered frame with a 30' foot trib and the outward kick on this needs some tie rods.

I designed the tie rods for strength first and then I went WOOOOOAAAAA, these suckers are going to elongate a lot.

Then I designed for the deflection and I went HEEEEEEEY! This is a lot of freeking steel.

Then I looked at previous jobs and saw that the documents made a note telling the contractor to tension the Tie Rods to produce a net lateral displacement at the baseplate and I went PHE

http://WWWWWWWW!

Now I can design for just the live load deflections.

The note says to tighten after all purlins, frames and decking are erected until each baseplate deflects 3/16 of an inch. But wouldn't it be better to specify a tension force? because what if the baseplates don't even kick out under the dead load as anticipated if the grout-baseplate interaction is extra strong that day (then your pulling extra hard and overcompensating). Or does this grout going to act more like butter compared to the kick force making the supports more like a pin-roller at first?

Also, do you guys make them wait until all of that dead load is on (purlins and deck)? Or just the bare frame dead load. Don't they have to tighten the rods before the slab gets poured? then how do they pour the slab when the roof is on the building? These construction documents might be a bust.

What do the professionals do?

 

[DaveAtkins]

I have never worried about elongation of the tension tie. Typically I use reinforcing bars encased in concrete, per that design guide written by Jim Fisher for Butler Buildings.

DaveAtkins

 

[DaveAtkins]

I pulled out my "Foundation Design and Construction Manual", the guide I mentioned above, and found that you are limited to 24 ksi when choosing reinforcing for a tension tie. For a 135' length, this would equate to an elongation of 1.34", or 0.67" at each end of the tension tie.

This doesn't seem like a big deal to me.

DaveAtkins

 

[jike]

I have always been concerned about the elongation of the tension tie! If elongation is significant and pre-tension is not done, then it will weaken the frame! The PEMB folks design the frames as if there is no movement in the foundation.

I agree with your point that measuring tension is more reliable than measuring deflection.

Dave:

I am not sure how the elongation is affected when the ties are encased in concrete.

 

[audeuce02]

Dave,

Do you have the actual "Foundation Design and Construction Manual", or a copy of it? During one of my moves, I have misplaced mine and can not locate a replacement.

 

[haynewp]

They do design as if there is no movement at the base of the frames. Some engineers design moment resisting footings using passive pressure to resist the thrust. There will also be movement in this case to develop the passive pressure.

 

[Stazz]

DaveAtkins, The tie rods are specified as A36 which makes sense with your 24 ksi requirement. Do you know why they have this requirement?

 

[jike]

Stazz:

1) Create a frame model with loads
2) record moments, shears, etc.
3) displace base of cols 0.67"
4) see how it affects moments and shears in the frame
5) what percent is the frame weakened?

 

[BAretired]

jike,

Displacing each of the column bases outward by 0.67" would put tensile stresses on the inside of all three frame members. In the elastic range, this would affect the resultant stresses but the ultimate strength of the frame is unchanged according to the principles of Plastic Design.

BA

 

[haynewp]

Metal building member section dimensions often fall out of the plastic design range.