Horizontal thrust in rigid frames

[rittz]

We need thrust rods in a pre-eng rigid frame building ...span 60 ft unfactored thrust 40 kips.
Should I use Dywidag corrision protected or plain Dywidag concrete encased or Ordinary Grade 400 reinforcing bars (concrete encased)? Coated rebar is probably available but pricey. Hair pins are not the greatest idea in grade supported slab over plastic clay. Any thoughts welcome.

 

[minorchord2000]

We typically design in-slab hairpin bars along each rigid frame. These hairpin bars wrap around the footing and are extended into the slab on grade to distribute the forces into the slab.

 

[Ron]

Consider a post-tensioned slab...solve the clay issue and the thrust

 

[msquared48]

If hairpins will not work, just encase two to four straight tension-tie bars in a concrete grade beam and install it under the slab with a foot of cover. These bars can be welded to a plate placed behind the anchor bolts of each end of the frame, and spliced as needed (splices staggered). I have used this many times with no worries where the tensiopn is too large. You do not need Dwydag bars. Grade 60 will be fine.

Mike McCann
MMC Engineering

 

[hokie66]

Agree with Mike that the high strength bars don't give an advantage over normal reinforcing. The elongation of the bars is controlled by area, not strength.

Agree that hairpins are not a good solution, but realize that they are a quick and dirty answer in PEMB. I don't like to depend on the slab for resistance to column thrust, because you don't want the slab to crack, and the slab may be interfered with at some stage.

Like Ron, I like post-tensioned slab on ground construction, but only if the tendons are bonded. Unbonded tendons give poor crack control, as the slab can slide relative to the tendons. But again, I don't want the slab to be restrained by the columns. A PT slab needs to be able to shorten due to shrinkage and applied PT.

The best, but perhaps most expensive approach is to design the footings to take the thrust. A two pile/pier arrangent usually works.

 

[haynewp]

I don't believe ACI allows tension-tie members to be lap spliced (see 12.15.5 in 2005). Look at mechanical or welded splices.

 

[archeng59]

Use a concrete tie-beam between the columns. The tie beam is located below the slab and can be poured with the slab. 60 ksi rebar is ok. 4#6 should be ok for 40 kip thrust, but verify that. haynewp is correct about ACI not allowing lap splices in tension members. Mechanical splices should be used. Lenton or cadweld splices are good.