Steel Building Anchorage with Uplift Forces

[bgsmith]

I am engineering the anchorage of pre-manufactured steel columns to concrete slab/footings under the 2006 IBC. The reference to ACI 318-05 Appendix D notes that under moderate or higher seismic areas the steel failure must govern design. For uplift resistance I am having trouble meeting the code. Since the metal building company specifies anchor bolt number, size, and spacing I can't adjust the steel anchor capacity. Even with large embeddments and large footings I still cannot get the concrete breakout strength to exceed the steel strength. I am aware that the IBC also allows the anchorage to be designed as 2.5 times the actual loads instead of requiring the steel to govern. Increasing the load by 2.5 causes either the steel bolts to fail or makes it almost impossible to develop that much strength in concrete breakout. In the case of shear I have added hairpins to transfer the load into the concrete across the failure plane, but in tension I cannot develop the hairpins without creating very deep footings (more than 24"). The design is for a thickened slab edge with spread footings under columns. Any ideas? Thanks.

 

[civilperson]

Use u-bars to surround the embedded anchor rods or a grid of reinforcement to intersect the breakout cone of concrete around the anchor rods.

 

[prsconsultant]

Try DSAnchor software at the link below, it would help you to get all the governing design values for a given anchor pattern.

http://www.dimsoln.com/DimSoln/dsanchor.html

 

[vincentpa]

Your piers will be huge. ACI 318 appendix D is an abomination. I would use Dimsol to design the footing. It is expensive but is handles uplift which every other program except one (which I can't remember the name) does not. It is easy to use and fast. I tried their program for anchors. It is fast and good. They are expensive, about $1500 a pop. Doing the problem by hand is very time consumming. Tell your client that the piers will probably be larger than he is used to seeing based of the new code. They might react well if you tell them in advance. Most do. If you don't, they will get angry.

Be careful with hairpins in the slab to resist the shear. You will have control and contraction joints and if you do not have rebar to tie teh slab segments together, you cannot count on the slab to resist the shear. The book by Alexander Newman "Metal Building Systems" is a good book to have if you are doing Pre-engineered Metal Buildings (PEMB) a lot. He talks about design of foundations for PEMB's. He also teaches a very good course for ASCE on foundations. I recommend it. There are a lot of failures in Metal Buildings but most are related to the superstructure NOT the foundations. Appendix D is overly conservative but it is the law, so we have to use it. I use hairpins to resist both the shear and tension. It is the only way for the high uplift and shear loads in PEMB's. I also rely on 1/2 the passive resistence of the soil and design the footing for overturning and uplift. hairpins in the slab are scary. I have seen bulldozers run over the hairpins and rip them right out of the pier in the field. I have seen the haipins placed behind the anchor bolts instead of in front of the anchor bolts. I believe using hairpins in the slab to resist shear is dangerous. Just my opinion.

 

[lkjh345]

bgsmith:

I have run into similiar problems in the past. If a different configuration (size, spacing. etc) of anchor bolts will help, don't hesitate to call the PEMB suppilier and talk to thier engineer. I have found many to be accomadating to requests for modifactions to their standard designs if a reason is clearly explained to them.

 

[JKW05]

Vincent,

Your post says "I use hairpins to resist both the shear and tension. It is the only way for the high uplift and shear loads in PEMB's"; then your last sentence says "I believe using hairpins in the slab to resist shear is dangerous."

I apologize if I'm missing something here, but where are the hairpins located, if not in the slab?

Thanks,

JKW

 

[vincentpa]

When I say hairpins, I mean hairpins within the pier and footing to resist internal forces in the pier. When I write hairpins in the slab, I mean hairpins protruding out from the pier or footing into the slab. Unfortunately, they are called the same thing.
Unfortunately ACI gives little guidance on the use of supplemental reinforcement to carry all of the load. They hamstrung with the Appendix D and then punted when there are serious questions. Only one guy really knows what the deal is and he is in Florida I think.

See these threads:

http://www.eng-tips.com/viewthread.cfm?qid=172687&page=2

and

http://www.eng-tips.com/viewthread.cfm?qid=185091&page=2

 

[vincentpa]

Also,

http://www.eng-tips.com/viewthread.cfm?qid=198887&page=1

 

[bgsmith]

Vincentpa,

To make sure I understand...you use vertical U bars facing down placed in the concrete pier under the steel column to resist the uplift forces? I have explored this option however to develop the hairpin into the concrete requires fairly tall piers (18" development + 3" clr + 8") approx 30". For contractors that are use to setting the bottom of their footings at 18" there is going to be some resistance. This also requires you to stop the anchor bolts fairly close to the top instead of how we have been typically detailing them as embedding into the footing. Am I understanding this correctly?

Thanks for the links to the additional threads relating to the topic.

 

[JedClampett]

Hairpins are traditonally horizontal bars used to increase shear capacity.
When you say you're having problems making the bolts work, what's controlling? I'm curious. I'm working on the same thing right now, but I think I can make it work. Is it edge distances, spacing or what?

 

[bgsmith]

It is really bolt spacing and size that is killing me. To make ACI the most efficient the bolts need to be sized to take just a little more than the applied load to ensure steel failure. Typically the bolt design is not adjustable by the foundation engineer where we do work.

Here is what I have:

6- 1" dia. bolts spaced 3" o.c.
using hef = 11"
(0.75)(0.75)Nsa=119.03k
(0.70)(0.75)Ncbg=31.92k f'c=2500 psi, Nb=43.53k Anco=1089in^2, Anc=1521in^2 (no edge within 1.5*hef)
(0.70)(0.75)Npn=259.78k

governing strength = 31.92k > 31.0 (factored uplift force)
however the steel does not govern so IBC requires you to increase the applied force by 2.5

With that increased load the anchorage would require approx. 22" of embedment. I am embedding this into the footing because if I use the depth of the concrete slab for embedment I have to use edge distance reductions and I haven't been able to ever get that to work. If I tell the contractor they need to pour 25" thick footings they will put me into the footings.

 

[msquared48]

Pre-Manufactured Metal buildings have an inherent tendency to want to fly, like they were designed by an aeronautical engineer. Unlike most other buildings, most literally have to be held down to the ground with extra concrete.

I am not at all surprised with your 25" thick footing, and have seen many more a lot thicker. If your contractor has a problem with that, tell him that you have to comply with the IBC Code and that's what the numbers say, but he is welcome to review your calculations or get a second opinion ... after you get your check. I have not had one do either yet. They just build it.

No worries.

Mike McCann
McCann Engineering

 

[vincentpa]

You are correct with when I am talking about vertical hairpins, I am using inverted U-bars with the hooks in the footing. Do not place the anchor bolts into the footing. If you really want to make a contractor mad, put the anchor bolts in the footing. It is impossible to construct and set the anchor bolts properly. The development length you are talking about sounds right. I am in PA and our footings go down 42". A 25" thick footing is too thick. Just make the footing wider and use part of your frost wall as dead load to resist uplift. You can try and try to make your pier and anchor bolts work for shear and tension with bolt spacing. It never will. You will have a 4'x4' pier. Believe me. I've wasted a lot of time trying!!! Remember, to use h effective instead of h. Too bad for your contractors if they are used to putting the footing 18" down. You have to drop your footing to accomodate your anchor bolts. The only way to make it work is to lap the bolts with hairpins into the footing. To make the shear work you have to use supplemental reinforcement around the anchor bolts. See AISC Design Guide No. 7 for more information on the design of both of these. Also, if you took ACI 318 course with the class notes. I am super busy right now or I would post a detail. It is the holiday crunch and I am off to Puerto Rico for Christmas and the New Year!

 

[vincentpa]

sorry for not being able to scan something for you guys. It is the best I can do without drawing a picture. Not many people have good answers for these things. I even spoke to someone on the committee for appendix D and he was stumped by the questions about shear I threw at him. I recommend AISC Design Guides 1 and 7, the second editions. They are free if you are a member. They explain it the best. there is also an old paper by someone or other that anchor bolt design was based on for 20 years. Can't remember the author though. It is referenced in ACI.

 

[bgsmith]

If you use hairpins and supplementary reinforcement to develop forces in your footings does the ductile steel failure requirement of ACI 318 D3.3.4 still apply? I know this appendix only deals with un-reinforced concrete anchorage but it doesn't seem like adding hairpins or additional steel reinforcing meets the intent of the section. Wouldn't the failure mechanism of reinforced anchorage require the concrete cone to develop and then transfer the load into the steel reinforcing? This doesn't meet the intent of a ductile steel failure does it?

Sorry about all of the questions and thanks for all of your input everybody.

Anybody else really enjoy designing anchorage with the Appendix D of ACI?

 

[vincentpa]

AISC Design Guide 7, first and second edition talks about this. The ACI code is too restrictive and AISC gives alternate advice.

 

[JLNJ]

It seems to me that if you can drive the failure into the verts in the pier then you have your ductile failure. You just need to make sure the bolt force can be developed into the rebar and the rebar are develpoed into the pier and footing.

Don't be afraid to apply engineering judgement. Without it all we are is code-mashers.

 

[spats]

To bgsmith - you need to grow a backbone. Why would you say "If I tell the contractor they need to pour 25" thick footings they will put me into the footings"? Who is the engineer-of-record, you or them? Who's putting their professional license on the line? Two foot thick footings are not unusual at all for high uplift loads.

If the anchor bolt size and pattern does not work, ask the metal building guy to change it.

You also indicate in your initial post, that "The design is for a thickened slab edge with spread footings under columns". Is this the contractors "requirement"? I don't use thickened slab foundations for anything but the smallest uplift loads, since the center of the column is almost always off-center from the center of the thickened slab. This requires bending resistance in the slab for stability, and the slab is almost never properly designed. I fight this from time-to-time with contractors, but I try not to fight with unarmed people. YOU ARE THE ENGINEER!