Aisi E 1, Volume Ii, Part Vii Anchor Bolt Chairs
Latrisha Adan
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In a photo posted in one of the articles that we published recently, it can be seen that one of the anchor bolts of a tank is broken or has failed after an earthquake and one of our readers asked us to please explain how this is possible if said bolts are there to hold the tank.
Fy = is the minimum yield strength of the anchor bolt or strap; bolts are limited to specified material minimum yield strength or 55,000 psi, whichever is less, in psi; anchor strap material minimum yield strength shall not exceed the minimum yield strength of the shell;
If this is not fulfilled, there is the possibility that the anchor chair, At the moment of being exposed to some event such as those indicated in table 5.20, it will be detached from the shell tearing the same, causing the contents to spill.
. distribute the load to the shell. Small tubular columns (less than 4 ft in diameler) may be an exception if the base plate is adequate to resist bending. Otherwise, chairs are always needed to minimize secondary bending in the shell.
For flat-bottom tanks, choose a bolt circle to just barely clear the bottom without notching It. For other structures, follow the minimum clearances shown in Fig. 7 1 a. The designer must evaluate anchor bolt location for interference wilh base or bottom plate.
Top Plate Critical stress in the top plate occurs between the hole and the free edge of the plate. For convenience we can consider this portion of the top plate as a beam with partially fixed ends, with a portion of the total anchor bolt load distributed along part of the span. See Fig. 72.
Chair Height Chair must be high enough to distribute anchor bolt load to shell or column without overstressing it. If the anchor bolt were in line with the shell the problem would be simple - the difficulty lies in the bending caused by eccentricity of the anchor bolt with respect to the shell. Except for the case where a continuous ring is used at the top of chairs, maximum stress occurs in the vertical direction and is a combination of bending plus direct stress. Formulas which follow are approximations, based on the work of Bjilaard.
Maximum recommended stress is 25 ksi. This is a local stress occurring just above the top of the chair. Since it diminishes rapidly away from the chair, a higher than normal stress is justified but an increase for temporary loads, such as earthquake or wind is not recommended. The following general guidelines are recommended.
If continuous ring is used, check for maximum stress in circumferential direction. considering the ring as though it were loaded with equally spaced concentrated loads equal to Pe/h. Portion of shell with in 16t either side of the attachment may be counted as part ol the ring. (Refer to Fig. 7-3)
Note that the base plate or bottom Is also subjected to this same horizontal force, except inward instead of outward. This is true even if a continuous ring is not used around the top of the chairs - but it should never cause any very high stresses in the base, so we do not normally check it. However. it is a good thing to keep In mind in case you have a very light base ring .
Where base or bottom plate is not welded to shell in the shop, as for flat-bottom tanks and single. pedestal tanks,. shop attach side plate.s to top plates and then ship the assembly for field installation. When you do this, weld both sides at top of side plates- so shrinkage will not pull side plate out of square. See Fig. 7-5.
Assume a stress distribution as shown In Fig. 7-6 as though there were a hinge at bottom of chair. For the purpose of figuring weld size, the base or bottom plate is assumed to take horizontal thrust only, not moment.
Note that loads are in terms of kips per inch of weld length, not in terms of kips per square inch stress. Critical stress occurs across the top of the chair. The total load per inch on the weld is the resultant of the vertical and horizontal loads.
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