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re: lap splices

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From a numbers/equation standpoint, the formula in ACI for lap splice length is roughly (without getting into all the nitty gritty factors and conditions):


Bar yield stress * diam bar * other factors (coated bars/ normal wt concrete)

(25 or 20)* sq rt of compressive strength of concrete



So increasing the concrete strength decreases your required lap strength, albeit in an “inverse square” relationship so it is of a diminishing return. I think I said that properly.


From a mechanics background perspective, it is related to the splitting strength of concrete, which is related to tensile strength, which is approximately 10-15% of the compressive strength . From what I understand before the 1970s there were flexural bond and anchorage bond requirements. But that did not properly address issues with splitting, specifically in situations where bars had little cover.


This makes for some pretty good bedtime reading if you are tossing and turning, ACI R12.1 Commentary….  There are three solid pages in ACI dedicated just to tension lap splices, but for most everyday situations you can boil the lap splices down to some very basic lengths. Most offices have a note on their dwgs that cover these situations. There are many others on this list who are more qualified to speak on this subject but that is a quick and dirty version.


Andrew Kester, PE

Orlando, FL