Lap Splicing Concrete Form Ties 1

[Bridge100]

A contractor has been successfully using lap-spliced snap ties in certain wall forming applications. I questioned the practice recently because it seemed there would be significant yielding from the eccentricity on the small diameter rods. They sent their modified products for testing and the product failed at 2 times the published safe worked load. ACI347-04 Table 2.3 simply states that formwork accessories are required to provide a 2 to 1 "safety factor based on the ultimate strength of the accessory when new." Are laboratory tensile tests enough to qualify this product as adequate based on the ACI requirements or must the tie be satisfactory per its calculated tensile rupture strength? If the latter, the effective net area would obviously be reduced by some amount (shear lag factor, U) below what the manufacturer designed for. Quick calculations (using AISC Table D3.1 Case 2 for Shear Lag Factor) based on photo estimated lap length and bar diameter tell me that this reduction is small. Can the yielding be ignored? That is not something I am used to doing. I have included a detail of what a snap tie looks like and photos of the test specimens.


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1710860091/tips/A-3_Standard_Snap_Tie_f88txu.pdf[/url]

 

[Enable]

Very neat. Thanks for sharing. Random thoughts:

1. This looks like a typical lapped rebar situation. In which case I think you'd use the same process for quantifying the connection. Find the capacity of the weld based on codified throat, account for eccentricity in weld capacity, and reduce net capacity of snap ties by some amount for shear lag. Eyeballing the dimensions I get a shear lag reduction of around 0.10 (U=0.90). If this causes the need for more ties just add them in. If it's a pre-built system with tie locations already fixed then either increase weld length to essentially eliminate shear lag, or move on to thought 3 below.

2. Yielding in this case really only presents two concerns: how much movement occurs in the form, and can the product be re-used. Since it's a snap-tie we don't care about re-use (single use product). Presumably the tests indicate how much yielding occurred prior to failure or you may even have the full stress-strain curve from the tested specimens. From this you can figure how much movement in the form you have until failure, and can compare it against allowable tolerances for wall formwork. If it's within the allowable range then I don't see a problem as long as the contractor understands this eats away at their tolerance.

3. You can't rely on the mean or lowest capacity observed in only a few tests if the numbers are way off what you'd otherwise get by calculation. But you can use statistical analysis to aid the cause. In this case I'd run a Bayesian analysis using the limit state calculations to inform the prior and see what the posterior predictive credible intervals returned. Two or three tests are probably not going to move the needle much from the prior but they might depending on how different they are. If this sounds like gibberish I can run something quickly for you if you provide the data / material and setup particulars. It's at least a rational method!

 

[Bridge100]

Thanks Enable! Sorry for the really late reply!! I hadn't posted a thread in a long time and my email address needed to be updated. Therefore, I never received a notification. It is always useful to get second opinions for the uncommon situations that we encounter from time to time. I will recommend to the contractor a lap length that will eliminate the shear lag reduction and a calculated estimate of the wall movement for your tolerance point.