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.
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Pre-tensioning the bolts to 75% of the allowable stress would be refering to the initial preload on the bolt. This has a fairly significant effect on the base and compression ring thickness.
Thank xema77. But I am confused with the positioning of the statement which is right after the given allowable tensile strength. Also as I understand giving allowance is some kind of safety factor. Bringing down initial preload from 100% to 75% only makes thicknesses in the basering and compression thinner. Please help me undertand. Thanks!
Structural engineers think of preloading (pretensioning) as those steel tendons in prestressed concrete design. There the steel tendons are stressed past the yield point to ensure that the concrete (weak in tension) is always subjected to compression and to reduce the deflections.
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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I agree that preloading (pretensioning) of vessel anchor bolts to a specific percentage of yield, for the reasons described above, is seldom done. It has been my experience however, that larger diameter vessel anchor bolts are tightened as much as possible using a hammer wrench or snipe. This would definetly subject the anchor bolts to some value of stress or preloading, although the value is not known. Therefore I would think that conservative value of initial preload should be specified. Any thoughts are appreciated.
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Pretensioning of Anchor Bolt