Pin or fix support?
Pin or fix support?
(OP)
for a reinforced concrete piperack supported by isolated foundations and no tie beams, what is the best assumption to represent my foundations? will it be pinned or fixed support? is there a big difference between pin and fixed in terms of design forces on the structural members and
foundations?
foundations?






RE: Pin or fix support?
There can be a big difference, depending on the geometry. Take just a simple beam, for example. Pinned connections give maximum moment at the center of the span, with tension at the bottom. Fixed connections give maximum moment at the ends of the beam, with tension at the top, with different moments from the pinned case. If you use the wrong end conditions, you won't even know what direction the moments are acting.
RE: Pin or fix support?
Refer to Foundation Design by Wayne C. Teng (1962) On pp 141-142 he outlines a technique for estimating the roatation of a footing under a moment load.
Also refer to Foundation Analysis and Dedign (fourth edition) by Joseph E. Bowles. On pp261-264 he outlines a method for calculating rotation of a footing under a moment load, and also a reduced moment on the column after rotation. He has an example.
Regards,
chichuck
RE: Pin or fix support?
RE: Pin or fix support?
RE: Pin or fix support?
RE: Pin or fix support?
RE: Pin or fix support?
RE: Pin or fix support?
1.If the column is cast-in-place and the foundation is massive and rotationally restrained (significant section and confined in the subgrade, no matter it is footing block, caisson or pile), the support is closed to “fixed”. Once you have the same reinforcement across the interface, the EI of the interface is the same as the column section. Other than under the seismic loading, unlikely the interface could work in a “yield” state. Even after yielding, the support is not really "pinned" since it continues holding a moment of My. In addition, the section cannot be underreinforced, we usually use in the range of 1 to 3 percent.
2.If the column is precast, the support fixity is dependent on the interface detail. You can make it close to “pinned” or “fixed”. Of course, foundation restraint is another factor.
3.It it always advisable to use “pinned” condition to check the extreme displacement while use “fixed” to catch the most critical internal forces at the interface.