tie rods for rigid frames 2

[syrupmaker]

I am designing a foundation for a 155' clear span rigid frame. I need to design a tie rod to carry the horizontal thrust at the base of the frame. What is the best way anyone has seen this done. Please be specific in your response concerning the load path from the column baseplate to the tie rod and how the tie rod is actually fastened to the concrete or coulmn. Thanks in advance.

 

[FFIELD]

I allways transfer the lateral load from the column to the concrete by means of the anchor bolts or shear plates. Then I embed a bearing plate near the anchor bolts (useally a large angle) and run the tie rods through the bearing plate with a nut on either side. Use sufficient reinforcing in the concrete to confine it in the load transfer area. Never had any problems.

 

[DaveAtkins]

I typically use standard hook bars in the pier, extending out into the slab and lapping with bars which extend to the opposite face of the buiding.

DaveAtkins

 

[Lutfi]

I do similar to what Dave does. I use hairpins. I use one set per two anchor bolts. I bend them to form a "VEE" with flat segment around the anchor bolts. The length of the hairpins depends on the tensile force to be resisted. You can bend the legs at 60 or 30 degrees. Art corners, I bend one leg so it is parallel with the building side. This theory assumes the slab to resist the horizontal thrust. I have designed many buildings using it and it works successfully.

I plan on writing a paper with a design example on how I design the hairpins. As a matter of fact, I have done so many; I have developed a table with various values. Some metal building suppliers used to issue similar tables in their literature. I think they are shying away or getting cheap.

Good luck

 

[syrupmaker]

Thanks, I appreciate the response. It seems there is more than one way to take care of the thrust. The important thing--just take care of it.

 

[alexio]

A few contrary points on this issue:
I suspect that what you are really looking for is not the tie rod design, but the design of a foundation that can resist lateral column reactions. Tie rods represent only one -- and not necessarily the best -- approach. There are other solutions (see Newman, Metal Building Systems, McGraw-Hill, 1997). Some of them, such as moment-resisting foundations in combination with slab dowels, may be better.

The main problems with tie rods are:
1. They elongate under load -- and allow the foundations to spread. This may be OK for a flexible metal building but not for one clad in brittle masonry. The best but $$ solutions are to encase them in heavy grage beams (say 16" x 16&quotor to post-tension them.
2. The tie rods do nothing against resisting wind loads acting inward (again, the grade beams would help here).
3. The tie-rods must be protected against corrosion, so they are ofter enclosed in grouted sheaths ($$)or, again, in concrete grade beams.
4. ACI 318 requires mechanical connections at tension-tie members...so sorry guys, lap splicing them is not allowed. For the same reason, hairpins are a no-no code-wise, despite the fact that pre-engineered manufacturers push them as a cheap solution to a complex problem.
5. Last, and most importantly, tie rods, if used, should be located well below the slab (12", say) so as not to be cut during future renovation activities. It is only a matter of time before someone will cut the slab to replace some piping or conduits. For the same reason, hairpins should be avoided in slabs on grade.

 

[RRBegin]

Lufti-

You wrote:
"I plan on writing a paper with a design example on how I design the hairpins. As a matter of fact, I have done so many; I have developed a table with various values. Some metal building suppliers used to issue similar tables in their literature. I think they are shying away or getting cheap."

How's that design paper coming? I've been looking for information on designing haripins, and the metal building manufacturer's no longer have design handbooks available; so...how's that design paper coming? (hint,hint)

 

[bjb]

Mechanically splicing tie rods encased in concrete is not a big deal. You can use something like the Zap Screwlok, or you can use a welded splice with A706 rebar and and steel angles. Our office has had good success with these methods. I prefer the Zap Screwlok. It is important that the construction of the tie rods (especially any splices)be covered by the special inspection program. Because of the critical nature of these elements, we design them for twice the applied load. We always encase them in concrete for corrosion protection, and always burry them below the floor slab. It is true that using tie rods should not be a "knee jerk" reaction to resisting rigid frame thrusts, sometimes a moment resisting footing is better.