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Allowable load for anchor bolts

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A usual trend in design of anchor bolt is to use 0.55*Fy or 0.33Fu as an allowable stress which is equal to 21.6 ksi and 19.14 ksi respectively for A307 material. Also based on spacing of the bolts the failure criteria of the concrete is checked which in some cases reduce these allowable stresses and by increasing the distances this problem could be solved.
Based on UBC97 (table 19-D) and IBC2000 (table 1912.2) the allowable loads given for the anchor bolts are much lower than the values specified. For example for 1" anchor bolt base on gross section of the bolt with max. distance between bolts the allowable stress is 4.65 ksi which is less than 0.2*Fy. This condition exist for all other bolts and all the allowable loads with all different distances are in the range of 15% to 0.25% of Fy. It seems that these allowable loads could be increased by increasing the length of anchor bolts, but I couldn't find proper formulations. Please advise me about these allowable loads and the usual trend in the way that consulting engineer companies design the bolts.
Anyway, please consider that we want to use ASD method instead of USD.
Best Regards,
Ali Karimzadegan