Jrswett
Structural
- Nov 28, 2022
- 12
This is in response to the thread "Retrofit Holdown - Can't Use Epoxy" thread507-473250. You are correct that epoxy hold downs generally do not work in a typical 8" stem wall due to the concrete breakout strength and inability to have a ductile fuse. I discussed the HDU and similar hold downs acting as the ductile fuse with Simpson staff and their strength is also too high to make this work. The base strength of the hold down had to be multiplied by the safety factor, this is presumably governed by the fasteners yielding in the wood. Using the steel rod itself as a fuse is another possibility, this requires the additional multiplier beyond the yield strength up the to ultimate strength. It's typically not feasible to make this work.
The workable solution is to use a PAB anchor bolt. They drill all the way through the existing footing and cast the PAB in a new, relatively small, concrete block below the existing footing. Alternatively they can jackhammer out a section of footing wide and deep enough to cast a typical 5/8" SB in fresh concrete. Leaving the edges rough and the rebar intact, a 12" wide section should suffice.
For this reason when doing retrofit hold down anchor bolts I always try to minimize the number that are required. A careful examination of the demand / capacity ratio can show where slight changes to the architectural design can keep the increased demand / capacity ratio below the 10% CEBC threshold where shearwall upgrades are required. Or in some cases the CRC can be utilized which doesn't necessarily require any hold downs, however the whole structure must meet the CRC which is difficult. And when hold down anchors are required I will try and do the minimum necessary, use force transfer shearwalls around windows, use the minimum amount of shearwalls to keep the redundancy factor at 1 and the nailing at 3" OC or above, make the hold downs coincide at corners where feasible, and so forth.
Happy designing
The workable solution is to use a PAB anchor bolt. They drill all the way through the existing footing and cast the PAB in a new, relatively small, concrete block below the existing footing. Alternatively they can jackhammer out a section of footing wide and deep enough to cast a typical 5/8" SB in fresh concrete. Leaving the edges rough and the rebar intact, a 12" wide section should suffice.
For this reason when doing retrofit hold down anchor bolts I always try to minimize the number that are required. A careful examination of the demand / capacity ratio can show where slight changes to the architectural design can keep the increased demand / capacity ratio below the 10% CEBC threshold where shearwall upgrades are required. Or in some cases the CRC can be utilized which doesn't necessarily require any hold downs, however the whole structure must meet the CRC which is difficult. And when hold down anchors are required I will try and do the minimum necessary, use force transfer shearwalls around windows, use the minimum amount of shearwalls to keep the redundancy factor at 1 and the nailing at 3" OC or above, make the hold downs coincide at corners where feasible, and so forth.
Happy designing
