Pretensioning of Anchor Bolt
Pretensioning of Anchor Bolt
(OP)
One of our client requirement is to give allowance in the design to pre-tension the anchor bolt by 75% of the allowable tensile stress. This requirement was given after they have stated allowable stress (tension and shear) of SA307- B / SA193-B7 bolts.
I have some difficulty understanding this since I am quite new on this work. Does this have something to do with initial preload in bolts (both PV Elite and Compress has option in basering design with regards to initial bolt preload percentage). Or does this mean that i have to use only 75% of allowable tensile strength in determining the size of my anchor bolt.
I would appreciate your help on this. Thank you very much.
I have some difficulty understanding this since I am quite new on this work. Does this have something to do with initial preload in bolts (both PV Elite and Compress has option in basering design with regards to initial bolt preload percentage). Or does this mean that i have to use only 75% of allowable tensile strength in determining the size of my anchor bolt.
I would appreciate your help on this. Thank you very much.
RE: Pretensioning of Anchor Bolt
RE: Pretensioning of Anchor Bolt
RE: Pretensioning of Anchor Bolt
The purpose of preloading the vessel anchor bolts serves a different purpose and it is not necessary to load the bolts beyond yield. Preloading the bolts to some given percentage of their allowable stress reduces the cyclic stress range that the bolts are subjected to during wind loads. The wind applies a varying force on the vessel and anchor bolts, varying with the wind speed. The bolts are then subjected to a range of tensile stress for a high number of cycles. Theoretically, reducing the range of stress will improve their fatigue life. See the text "Pressure Vessel Design Handbook" by Bednar, or the little green book from ASCE ("Wind Loads and Anchor Bolt Design for Petrochemical Facilities").
In my experience and in observation of COMPRESS design files from customers preloading anchor bolts is not something that is typically done. Only the most important "tower"-type vessels will likely have this done. Just the procedure of preloading the anchor bolts will require special tools and training that is not typically available in the field.
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RE: Pretensioning of Anchor Bolt