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"). Any ideas? Thanks.
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Metal Building Anchorage to Concrete
- Thread starter bgsmith
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bgsmith, this is a problem we've had too. However, we weren't aware of the 2.5 safety factor option -- can you point me to the code reference on that?
Like you, we use hairpins for the shear. For tension we usually end up justifying it through planscheck with an argument that there's a huge safety factor compared to the actual required design loads. Most of the other options produce idiotic engineering results -- creating a weak point in the connection or bypassing Appendix D by using the items it lists that it doesn't cover.
Some other options we have actually resorted to are:
1. Increase the size of the footing width/depth -- 24" isn't that bad for a metal building footing. I'm surprised you're not that thick for just your required DL mass to resist overturning in the 0.6D + W load combo. That load combo usually controls the footing size for us and 24" would be on the low end of what we usually get for thickness in a metal building.
2. We have sometimes embedded a solid plate at the bottom with all the anchors attached to it. Appendix D has a specific exception that it doesn't include this type of connection. You have to design this more with rational engineering principles (bearing, shear-cone failure, etc) than any specific guidance I've seen.
3. We've considered using a bolted bearing plate to an embed plate in the footing connection and then using hooked rebar on the embed plate instead of anchor bolts since this isn't covered by App. D. To my knowledge, we've not had to resort to this.
4. Now, HILTI has an epoxy that might be an option now: HILTI RE500 SD. Check into that as I think it's the only epoxy with ICC approval for 2006 IBC. Don't know if that produces better results, but I don't remember seeing the requirement for steel-controlled design in their stuff.
Good luck and let me know if you came up with something creative.
Like you, we use hairpins for the shear. For tension we usually end up justifying it through planscheck with an argument that there's a huge safety factor compared to the actual required design loads. Most of the other options produce idiotic engineering results -- creating a weak point in the connection or bypassing Appendix D by using the items it lists that it doesn't cover.
Some other options we have actually resorted to are:
1. Increase the size of the footing width/depth -- 24" isn't that bad for a metal building footing. I'm surprised you're not that thick for just your required DL mass to resist overturning in the 0.6D + W load combo. That load combo usually controls the footing size for us and 24" would be on the low end of what we usually get for thickness in a metal building.
2. We have sometimes embedded a solid plate at the bottom with all the anchors attached to it. Appendix D has a specific exception that it doesn't include this type of connection. You have to design this more with rational engineering principles (bearing, shear-cone failure, etc) than any specific guidance I've seen.
3. We've considered using a bolted bearing plate to an embed plate in the footing connection and then using hooked rebar on the embed plate instead of anchor bolts since this isn't covered by App. D. To my knowledge, we've not had to resort to this.
4. Now, HILTI has an epoxy that might be an option now: HILTI RE500 SD. Check into that as I think it's the only epoxy with ICC approval for 2006 IBC. Don't know if that produces better results, but I don't remember seeing the requirement for steel-controlled design in their stuff.
Good luck and let me know if you came up with something creative.
civilperson
Structural
A grid of reinforcement at the top of the anchor rods will intersect the pullout cone in the concrete. Sufficient area of steel will make the tension strength of the anchor rod less than the pull out value of the concrete plus the reinforcement crossing the cone surface.
- Thread starter
- #4
ARKeng:
The 2.5 factor is in the IBC portion of our code (1908.1.16)that deals with concrete. I don't know if this is a state specific revision or general IBC revision. To resist the uplift forces I have been including a portion of the slab above which helps. You are right though that footing size is usually determined by the 0.6D + W load combo. The problem is in my area the bedrock is very close to the surface so excavation cost are huge if you get very deep. I have also installed a plate on the end of the anchors in an attempt to expand the concrete breakout cone but I still couldn't get much capacity and the thickness of the plate to resist the bending got a little large.
The ductility problem still arises in shear. Even if you use a #6 hairpin at each row of bolts you cannot get enough strength to exceed the bolt capacity. How do you get the steel in the achors to fail first?
civilperson:
I have been trying to justify using the top grid of typical reinforcement in the isolated footings to resist the concrete breakout. I have been using the shear-friction equation from the ACI and then making sure the rebar has development length beyond the concrete cone intersection. I seem to get capacity values that are 3-4 times the values using ACI Appendix D. I don't know if this is a reasonable analysis procedure but I haven't seen anything better.
The 2.5 factor is in the IBC portion of our code (1908.1.16)that deals with concrete. I don't know if this is a state specific revision or general IBC revision. To resist the uplift forces I have been including a portion of the slab above which helps. You are right though that footing size is usually determined by the 0.6D + W load combo. The problem is in my area the bedrock is very close to the surface so excavation cost are huge if you get very deep. I have also installed a plate on the end of the anchors in an attempt to expand the concrete breakout cone but I still couldn't get much capacity and the thickness of the plate to resist the bending got a little large.
The ductility problem still arises in shear. Even if you use a #6 hairpin at each row of bolts you cannot get enough strength to exceed the bolt capacity. How do you get the steel in the achors to fail first?
civilperson:
I have been trying to justify using the top grid of typical reinforcement in the isolated footings to resist the concrete breakout. I have been using the shear-friction equation from the ACI and then making sure the rebar has development length beyond the concrete cone intersection. I seem to get capacity values that are 3-4 times the values using ACI Appendix D. I don't know if this is a reasonable analysis procedure but I haven't seen anything better.
I don't know the particulars but a friend of mine just used a Williams product to help with a steel building anchor problem under the new Florida Building Code.
Look into AISC Design Guide No. 1 and No. 7 second additions. They talk abou this issue. They suggest an alternative to the steel yielding first. It is almost impossible to have the steel yield first with PEMB. Appendix D is an abomination. Some people have political causes. Some people are nuts about the environment. Mine is Appendix D!!!!!
I hope this helps.
I hope this helps.
BGsmith,
I have been doing what you have using shear friction also. I develop the rebar on both sides of the potential crack for shear and tension. It gets crowded up top. I also use vertical hairpins, that is I use long anchor rods and lap them with the vertical steel to take out the tension. You can't put the anchor bolts into the footing if you are pouring a pier on top of the footing. The contractor will scream justifiably that he won't be able to set the anchor rods properly by pouring the footing and then the pier. I have many details and suggestions I will scan some day and post on here because this is a big problem for us.
ARKeng,
I wouldn't put a full plate on the bottom of my anchor rods because it creates a weak plane in the conrete. AISC and ACI advise not to do this.
I have been doing what you have using shear friction also. I develop the rebar on both sides of the potential crack for shear and tension. It gets crowded up top. I also use vertical hairpins, that is I use long anchor rods and lap them with the vertical steel to take out the tension. You can't put the anchor bolts into the footing if you are pouring a pier on top of the footing. The contractor will scream justifiably that he won't be able to set the anchor rods properly by pouring the footing and then the pier. I have many details and suggestions I will scan some day and post on here because this is a big problem for us.
ARKeng,
I wouldn't put a full plate on the bottom of my anchor rods because it creates a weak plane in the conrete. AISC and ACI advise not to do this.
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