Steel Building Anchorage with Uplift Forces
Steel Building Anchorage with Uplift Forces
(OP)
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.






RE: Steel Building Anchorage with Uplift Forces
RE: Steel Building Anchorage with Uplift Forces
http://www.dimsoln.com/DimSoln/dsanchor.html
RE: Steel Building Anchorage with Uplift Forces
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.
RE: Steel Building Anchorage with Uplift Forces
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.
RE: Steel Building Anchorage with Uplift Forces
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
RE: Steel Building Anchorage with Uplift Forces
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
and
http://www
RE: Steel Building Anchorage with Uplift Forces
http://www
RE: Steel Building Anchorage with Uplift Forces
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.
RE: Steel Building Anchorage with Uplift Forces
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?
RE: Steel Building Anchorage with Uplift Forces
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.
RE: Steel Building Anchorage with Uplift Forces
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
RE: Steel Building Anchorage with Uplift Forces
RE: Steel Building Anchorage with Uplift Forces
RE: Steel Building Anchorage with Uplift Forces
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?
RE: Steel Building Anchorage with Uplift Forces
RE: Steel Building Anchorage with Uplift Forces
Don't be afraid to apply engineering judgement. Without it all we are is code-mashers.
RE: Steel Building Anchorage with Uplift Forces
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!