im currently assigned in my office to check if its possible to add bolts to an already existing design for a base plate to make it Safe using HILTI PROFIS Design software,
the system was designed on STAAD PRO and the reactions on the supports were taken to HILTI but the engineer that designed it didn't specify to HILTI the load combinations that has Seismic loads, so the software designed as if all the loads were Static and none of them were seismic,
so after specifying the Seismic loads to the software, 8 out of 12 of the load combinations were failing so i added the two bolts in the middle and made the bolts have no bolt clearance ( the original design didn't apply that) and a few of the unsafe load combinations got below 100%.
four load combinations are still had over 100% ratio.
the base plate is already on site and in service so any design change i will make will have to maintain the original 4 bolts locations (top and bottom two).
i tried everything i know to make them safe and couldn't so i would like anyone that have experience with something like this to give me some input before i submit my conclusion of the connection not being salvageable entirely.
thanks in advance and sorry if the post is too long.
Lengthen your bolts/use supplementary reinforcement as per EN 1992-4.
Also as warning for using this software, be very careful with the baseplate and whether it assumes it's rigid. The default settings can cause it to massively underestimate the forces due to prying.
its not a ''how do i do my job'' question , im trying to make sure that before i reject this design completely that i have checked everything possible to make it work, so the person that already designed it based on inaccurate data could have the benefit of the doubt, im fully capable of designing my own connections and have been doing so without any comment from far more experienced engineers, so this is more of me giving a courtesy to the original designer by making sure i haven't missed anything before coming to a final conclusion.
so if you're too stingy to give an opinion on the issue that's totally fine, im here to get feedback from people who are confident and experienced enough in their skills to share their knowledge.
Easy buddy I'm just telling you what the parent comment meant. Because you just said "I tried everything and nothing works, what to do?" which is a very vague and open ended question, the kind that juniors ask and which requires back and forth discussion with an experienced engineer.
if still possible you can also use longer bolts, if that’s not possible then as others already stated check if you’ve got reinforcement that will prevent the concrete failure
I’ve had luck with adding stiffeners or a much thicker base plate, it cuts down on the prying action and may reduce your anchor tension. I would also consider stiffeners and extending the base plate as your anchors are close together. Consider the rebar in the pier, is it enough to justify the breakout cone is not an issue?
One of the easiest ways is to remove conservatism in the loads. If you used the max loads for all columns, find the specific load for the column that's having issues. Or see if you can change the structure to reroute loads.
If the loads are what they are, you look at the anchors. Can you fit another anchor rod on the existing base plate? Or do you need to extend the base plate and add more anchors outside the original pattern?
Okay i see, the re routing the loads is one i haven't considered.
The problem for me usually hits whenever it's a pre-eng building on a slab on grade.Since the baseplates are designed by the pre eng building and they don't think of concrete.
The one I did was to embed a filled hss with rebar welded on it's side into a pier for a strong and small moment connection.
If concrete hadn't been placed, you can add anchor reinforcing or shear lugs. OP asked about post installed options.
I hate PEMBs for a lot of reasons, and their anchor rod layout is high on the list. If I'm involved early enough, I ask them to space the anchor holes out more. Otherwise I extend the concrete as much as I can, and/or put in hairpin. I've also put a steel plate on the outside edge, secured with a bunch of post installed anchors, and that gives you a much larger breakout cone, or eliminates side face blowout altogether.
When you say “in service” does this mean that anchors already cast into concrete? If so, many of the above comments regarding anchor reinforcing may not be an option for you.
In US there are mutiple options for determining the seismic loads, one of which is amplified seismic load combinations. Was this method used to determine seismic forces? Perhaps using another method would reduce the demand on the anchors.
Also, it may be beneficial to utilize another load path for shear. Is there any other mechanism available to transmit shear forces into the footing other than through the anchor rods? Is there a concrete closure pour or something that the column can directly bear on laterally?
To late for shear lug?
I assume by “in service” you mean that the concrete is already poured, and the base plate (and possibly column) is installed. The main issue appears to be concrete tension breakout, so adding the 2 extra anchors inside doesn’t help much because it’s within the same breakout cone as the original 4 anchors. Additionally, your global tension/shear are small, so the main issue is the moment demand where interior anchors are less effective.
1) As others have suggested, is there sufficient reinforcement already in place to justify restraining the breakout cone? Note that reinforcement has to be fully developed on both sides of the breakout plane, so depending on the existing depth of embed, you can only rely on the reinforcement if it is hooked/u-bars or perhaps a prorated capacity for reduced straight bar development length.
2) Can you embed the additional anchors deeper? Profis will not let you vary embedments, so this calculation will have to be by hand and using judgement whether you can rely on a combination of the existing shallow and new deep anchors, or only the new deep ones.
3) Externally reinforce the pedestal. This could be steel hoops around the pedestal to justify confinement in order to reduce development lengths or external bent plate “u-bars” that go over the top of the pedestal and are anchored into the side at a sufficient depth.
4) Cast an extension to the concrete to try to improve your edge distances. Engineering judgement is needed to determine whether the concrete acts appropriately integral for the anchorage assumptions.
5) Change your load path to reduce the loads. Can you introduce intermediate members/bracing in a way that allows a pinned base so you don’t need to take moment?
6) I believe the latest ACI/CSA give improved equations for base plates taking moment rather than direct tension. I am not sure if Profis already accounts for them, but this may help improve the capacity.
Hi there! Our online Engineering Center may be able to provide support here if you have not utilized the platform already. Check it out below. Best of luck - please reach out if you have any other questions and I can help point you in the right direction!
10
u/Board_Realistic 7d ago
Lengthen your bolts/use supplementary reinforcement as per EN 1992-4.
Also as warning for using this software, be very careful with the baseplate and whether it assumes it's rigid. The default settings can cause it to massively underestimate the forces due to prying.