Horizontal thrust in rigid Frames ... pre-engineered steel buildings

[rittz]

What kind of rod do you use? and how do you protect it from corrosion? When would you use hairpins in the slab for thrust resistance instead of rods?
BTW We use cast-in-place conc slab and grade beam for foundation. More often than not we are in high plastic clay in this area.

 

[Ron]

ritz....are you asking about rods in the plane of the walls or the roof? Horizontal loads can be helped by either or both. Is the thrust from wind or crane loads?

I'm a little confused by your question. Maybe I'm just having a dense moment, but please explain.

 

[dik]

I would think that these are horizontal rods in the plane of the floor slab to resist horizontal forces from the PEMB. If an inside environment, then corrosion is not likely an issue and I would normally use regular rebar. If corrosion is an issue and HDG rebar satisfy the corrosion type, then use it... else use something that is resistant.

Dik

 

[rittz]

Ron
See attached sketch

 

[hokie66]

With tie bars under the slab, you could do it a few ways:
1) Cast the bars into a concrete tie beam to allow reinforcement development. Separate from floor slab.
2) Use tension couplers on the bars, then perhaps wrap with Denso tape.
3) Use strand in plastic duct, stress and grout.

 

[dik]

I often use a 'hairpin' around the anchor rods or a steel angle (drilled for the anchor rods) with anchors welded to them and depending on the loads and span sometimes anchor the ties into the slab and use the slab rebar to transfer the load.

Dik

 

[Ron]

ritz...thanks...sorry for the lapse.

We use similar to dik's description....sometimes hooked dowel bars from footing into slab in addition to hairpins...we don't often use grade beams for PEMB's.

 

[civeng80]

Hairpins anchored with ties to the slab is very ofetn used but it cracks the slab.

As Hokie66 says it should be seperated from the slab.

 

[DaveAtkins]

Hairpins are my first choice--but there is a limit to how much load they can take, based on reinforcement in the slab-on-grade or friction under the slab.

For larger thrust, I use reinforcing bars encased in concrete, under the slab. I detail hooked bars extending out of the column pier and lapping with the bars under the slab.

DaveAtkins

 

[WillisV]

DaveAtkins - For the larger thrust conditions, since the tie-bars are in constant tension due to the thrust loads, how do you justify using a standard lap splice? (ACI requires a mechanical splice for reinforcing in a constant state of tension).

 

[vandede427]

for small thrusts you can do without hairpins. you just have to crack open the ultra-stupid Appendix D.

 

[rittz]

How much would a "small" thrust be guys? >40kips?

 

[OHIOMatt]

One additional thing to consider is the use of the building and the potential for slab repair or replacement in the future. If you use hairpins, there is a chance, that they will be removed during a slab replacement or the addition of new underground equipment or piping.

Hokie66's described method is what I try to use. It reduces the chance of the above and also reduce cracking in the slab.

 

[Teguci]

Couple of things regarding tension ties in SOG:
1. To account for differential settlement between the column foundations and the concrete slab a length of pipe insulation can be wrapped around the tie bar before being cast into the slab. This will allow vertical displacement without cracking the slab.
2. Because structural loads are being placed in the slab, ACI 318 design is mandated. Standard SOG design will not pass muster for US jobs. This includes rebar spacing, clearances, and shrinkage reinforcement.

 

[DaveAtkins]

WillisV--

I have always used Jim Fischer's recommendations in the Butler Foundation Design and Construction Manual. He feels this tie below the slab is not really a reinforced concrete member, and therefore is not covered by ACI. Also, extensive testing has shown that lap splices surrounded by stirrups can take repeated loads.

rittz--

The Butler Manual limits hairpins to 27.4 kips (this would be with a #8 hairpin).

DaveAtkins

 

[rittz]

WillisV
Butler Canada tells me the Butler Foundation Design and Construction Manual has not been printed for quite some time. Any chance a copy may be available in the US. Is it many pages or reasonably scan able? Thanks for the info you posted in any event. We appreciate your response

 

[msquared48]

Any lateral kick where I need more than 2 #5 hairpins, I switch to under the slab continuous ties, and I weld these ties together at the splices with an angle iron splice, all embedded in a concrete grade beam.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vandede427]

"How much would a "small" thrust be guys? >40kips?"

I would consider >40 kips to be a very very large thrust for PEMB.

When I suggested not using hairpins for small thrusts, I was thinking in the <8 kip range. Sometimes I run grade beam reinf cont thru the footing and use that as confinement steel and go without hair pins for higher thrusts.

 

[rittz]

We have used Turnbuckles to join two tie bar rods which are anchored to the pile caps at each side. The tie bar is below and isolated from the slab. The urnbuckles can be pre-tentioned before the building is placed on the foundation to reduce total extention. We use weldable rebar. None of the posts mention this system. Anyone use this to resist thrust force?
Also regarding heave ... our main concern is from heave and subsidence (due to highly plasic clay) which varies with the seasons and thus with the moisture content. We seldom never use (read .. never )a strip footing with a strip "frost wall" or pad footings the cols and grade beam. We use a grade beam supported by cast-in-place concrete friction piles or driven timber piles or screw piles.

 

[hokie66]

rittz,
Yes, I have used that system of tie rods. See my post above. To the piling options you gave, I would add bored piles. Using two piles at each column gives you enough moment capacity to at least partially fix the base.