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Different Anchor Rod Tensile Strength between ACI 318 method and AISC method 4

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GEOzhen

Structural
Oct 21, 2019
5
Hello Everyone,

I have a question regarding the different methods for calculating anchor rod tensile strength using AISC 360 and ACI-318. I understand AISC calculating the anchor rod tensile strength using nominal diameter and ACI uses the net tension area. I found that the ACI method sometimes can yield a much higher capacity than the AISC method. Especially for the large diameter anchor bolts.

Taking a 3' diameter F1554 Gr.36 anchor for example, the AISC method yield a tensile strength of 231 kips. Yet, using ACI 318-14 equation 17.4.12 with 0.75 reduction factor, the tensile strength is 259.7 kips. ( The net tensile area is 5.97 in2 taking from AISC Manual 15th, Table 7-17) The difference is about 12 percent. Could anyone suggest what is the logic behind this? I saw AISC DG1 mentions both method should yield similar results. Am I missing something in my calculation?

Thanks in advance.

 
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As noted in Design Guide 1, AISC utilizes a simplified method that utilizes a modifying factor that relates the tensile stress area directly to the unthreaded area (0.75). As is typical, the simplified process is more conservative than the use of the equation to find the actual effective area of the bolt.

The reason that the simplified method gets more conservative with the large diameter bolts is that once you get to a 2-1/2" diameter bolt, the threads per inch no longer continue to decrease, they stay at 4 threads per inch up to a 4" diameter. The depth of the threads is directly related to the threads per inch (see table 7-17). So as the anchor diameters increase up to 2-1/2" the depth of the threads also increase. However, above 2-1/2" the depth of the threads remain constant, so the percentage of the effective area to the gross area continues to increase for each diameter larger than 2-1/2". The simplified AISC method does not take this into account as it uses a straight 75% factor.
 
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