If you would like to know more about the International Code Council’s new state-of-the-art retrofit guideline please type ICC 1300 in the search box.
Here are two videos that look at the ability of old unreinforced concrete to resist the upward pull caused tie-down hardware (also known as the Simpson StrongTie HDU2) that is only found in the seismic retrofit guideline ICC 1300 Vulnerability Based Seismic Assessment and Retrofit of One and Two Story Dwellings.
This 13-minute video looks at everything you might want to know about old concrete and the performance of tie-downs. The Simpson StrongTie name for the tie-down used in the retrofit guideline is the Simpson StrongTie HDU2. I will be referring to them by that name from here on. This hardware can only be found in ICC 1300, which is one of the reasons this guideline is superior to its predecessors. As you will understand later, the new seismic retrofit guideline allows this standard to apply to cripple walls more than 4 feet tall. This is not the case with the other retrofit guidelines.
This 4-minute video looks at the basics of HDU2 holdown installations in old concrete. The essentials of this topic can be found here. You may want to start with this video first before looking at the more detailed video.
I suggest you go to this webpage and read the material and watch the video to understand what HDU2 holdowns actually do.
This is not an unimportant matter because the ten tables in P-1100 base the quantities of hardware and plywood required for each type and size of the house the tie-down tension resistance, and therefore on concrete quality.
There are ten tables with columns similar to the ones with the red arrows.
The tension resistance of existing 75-year-old concrete is suspect under the best of conditions, but using it to resist overturning is far better than doing nothing at all. On the other hand, it might be superior to new concrete. No one knows because no one has ever tested old concrete. Below is a table from ICC 1300 Vulnerability Based Assessment and Retrofit of One and Two-Story Dwellings. The tie-down shown is the Simpson StrongTie HDU2 and has 3075# of tensile strength.
Hilti analyzed the Tie Down detail and determined the anchor will provide 2344# in tension before the concrete breaks out in 2500 psi concrete. This is quite a bit less than the 3075# needed for the new seismic retrofit guideline. Then again, older concrete maybe even stronger than new concrete.
The California Existing Building Code assumes all existing concrete is 1500 psi so I extrapolated another 702# for a total of 1642#
Below are the comments from Hilti and Simpson. In the first edition of the ICC 1300 retrofit guideline, a torque test was suggested to check for concrete quality. This requirement was changed in a more recent edition but these letters are interesting nevertheless.
This letter from Simpson StrongTie addressed this original testing procedure and now the concrete quality is to be determined by the contractor.
This report from Hilti says the exact same thing. If you want to test your concrete I suggest drilling into it. The more difficult it is to drill, the better the concrete.
Based on the evidence in this video, some people might think tie downs aren’t a big deal anyway.
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