Are hairpins against the Code? 1

[TonyES]

I've got another engineer who has stated that hairpins are not allowed per the code. I wasn't able to get a clarification nor have I found anything that validates this comment but the client wants some proof of my design (using hairpins) before he accepts it. To clarify - my client has used another engineer for years - this engineer hasn't allowed hairpins for any of his designs so the client freaked out when I submitted my design with hairpins. I'm referring to pre-engineered metal buildings where I do solely the foundation design based on the manufacturers forces and calculations. It is a small building with relatively small thrust/horizontal forces.
any code locations would be greatly appreciated.
thanks

 

[rday]

I've never seen anything in any code that prohibits hairpins.

 

[bridgebuster]

Ditto

 

[Teguci]

Just two thing:
1. if you put structural loads into the slab on grade, then the SOG needs to be designed to ACI 318. This includes clearances, shrinkage steel, etc...
2. For Seismic Design Category C or higher (IBC 2006 1808.2.23.1) the piers, piles or pile caps are to be connected using tie beams or you can transfer the load in the slab on grade.

 

[DaveAtkins]

Slabs-on-grade are not covered by ACI 318.

Almost everybody uses hairpins for this type of structure.

DaveAtkins

 

[dik]

I've even used single legged stirrups...

Dik

 

[SteelPE]

I love it when people ask me questions to which I can’t answer. I don’t think you are going to find a code provision that says you can use hair pins. You may find something that says you can’t use them but then again, if you knew about the provision you wouldn’t have designed the building using them.

FYI, I have yet to see any code provision that prohibits hairpins… except in certain flood zones (but that is a different animal). Of course I don’t sit down all day reading codes either. I have also seen other foundation designs that utilize them. I also have a book somewhere that shows you how to design them (or at least a few photocopied pages).

 

[Teguci]

DaveAtkins,
ACI 318 excludes the design of soil supported slabs "unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil." 1.1.6

I usually read this to mean that if you are transmitting lateral loads through the SOG (ex tie-back at the top of a retaining wall or lateral ties between foundation elements)then ACI 318 applies.

 

[Ron]

ACI 318 requires them for groups of tendons under certain spacing requirements and allows them in beams. As noted 318 does not specifically address the SOG condition.

 

[dakota99]

We have used hairpins in many metal building applications. To my knowledge it is not a code violation. I am studying for the structural exams right now and I have had allot of intimate time with the codes and have not come across anything.
I use a reference by Alexander Newman called Metal building systems when designing foundations for these buildings. Chapter 12 covers the design of foundations for metal buildings.
I will say that I am now tending to use moment resisting foundations for light loads. I have seen several buildings where the owner cut in trench drains or other equipment into the slab after the construction was complete.

 

[vandede427]

it's obvious the guy doesn't know what he's talking about. there's one thing to not use them, but you said he "hasn't allowed" them. that's even worse.

 

[JAE]

Teguci - I think you have a point with section 1.1.6.

However, Section 7.12 (shrinkage and temperature steel) does not apply to ANY slab-on-grade.

TonyES,
I wonder if the other engineer is thinking along these lines:
1. A hairpin is a bent bar that extends directly around the anchor bolt group.
2. Tension transfer into an anchor bolt group must develop the tension into the hairpin through direct contact with the anchor bolts.
3. Since ACI 318 recognizes transfer through development length of straight bars or hooks, the concept of transferring lateral force in vertical bolts through a bent bar that isn't developed l_dh past the shear plane doesn't properly happen.
4. Therefore, hairpins don't "work" via ACI 318.

Also - the concept of trenching through the slab, reliance on shear friction between slab and subgrade, etc. might be cause for worry in some engineer's hearts.

I agree with the other above that hairpins are used quite extensively. I've used them - but I've always asked about trench drains, future use, etc.

 

[TXStructural]

So long as the bend meets minimum bend diameter requirements and the legs develop required capacity with other reinforcing, there should be no problem with hairpin use. However, if you are using only hairpins, without reinforcing the remainder of the slab, there will be no load path for structural loads to move into or through the slab.

CRSI typical bar bend S11 is a hairpin; it is recognized in the current CRSI/ACI joint committee 315 report (ACI 315).

 

[msucog]

I don't know specifically, but in the areas I work (which are typically "by the IBC book"), I see them all the time for such structures...or more correctly did see them all the time prior to the recent collapse of the construction industry. That doesn't mean it is or isn't but that's my observations of current practices here.

 

[bones206]

ACI 318-08, Appendix D

Figure RD.6.2.9(a) illustrates the use of hairpins as "anchor reinforcement" for anchors loaded in shear.

"D.6.2.9 - Where anchor reinforcement is either
developed in accordance with Chapter 12 on both
sides of the breakout surface, or encloses the anchor
and is developed beyond the breakout surface, the
design strength of the anchor reinforcement shall be
permitted to be used instead of the concrete breakout
strength in determining ?Vn. A strength reduction
factor of 0.75 shall be used in the design of the anchor
reinforcement."

You can use this provision to defend the validity of your design.

See D.5.2.9 for the use of hairpins to resist tension loads.

 

[Lion06]

bones beat me to the reference to App. D.

Additionally, I don't think that it is incumbent upon you to find something in the code that explicitly allows hairpins. If he's making the statement that they're not allowed "per code", then (IMO) it's incumbent upon him to provide the code reference showing that the hairpins are not allowed.

 

[miecz]

ACI 12.15.5 requires splices in tension tie members to be made with a mechanical or welded splice. Perhaps the engineer considers the hairpin to be a component of a tension tie member.

 

[RacingAZ]

That's a good one, StructEIT. A star for you.

The burden is on the other guy to provide you the code reference where it says that hairpins are not allowed.