Pin or fix support?

[gaijin2004]

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?

 

[JStephen]

The model should reflect the rigidity of the object being analyzed. If it can carry a moment and a shear at the point where it attaches to the foundation, treat it as fixed. If only a shear, then pinned. There are other choices, too, those are not the only two possible end conditions.

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.

 

[chichuck]

gaijin2004,

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

 

[BFPartners]

As for your foundations, the question is about deflection and rotation. Two or four anchor bolts or just on a rubber bearing pad? How does the reinforcing steel from the frame connect to the base. Is the base significantly larger than the frame member? Will there be differential settlement between the footings? The moment diagram and shear diagram will direct your design efforts in terms of where the reinforcing in the frame is to be placed. These questions will help lead you toward the correct assumption of the base restraint and if you have deflection or rotation. Most basic textbooks in reinforced concrete design and soil mechanics will aid you. Conservatively use the worst case!

 

[J1D]

Analyze a typical bent to see whether you need the fixity at the bases. For a reinforced concrete piperack on isolated foundations, I would make the supports being "pinned" to minimize the bending moment to foundation. Piperack bent with rigid frame and pinned support is very common, not mention you have rigid concrete frame.

 

[Ron]

Design it either way! The only difference it will make is the member sizes/reinforcement at two different locations. If pinned, the rigid bent members will need to stronger at the beam/column interface. If rigid, the footing mass must handle the overturning moment and the induced moment. You can do it either way.

 

[gaijin2004]

Thank you guys for your valuable comments! We have been designing pipe racks (steel & RC) with pinned supports and i I got this question raised up when a new engineer in our office always use fixed support but cannot explain why it is so. Yes, we can design it either way but what would be the implication if for instance I designed a foundation based on a pinned support and then later check this amount of reinforcement based on a fixed support condition and will find out that reinforcement is not enough, will the foundation fail? Or do I have to check my foundation design for both conditions?

 

[Ron]

gaijin2004....keep in mind that the ultimate construction of an item dictates (somewhat) its design parameters. Just because we analyze something as pinned or fixed doesn't mean it actually acts that way in the constructed product. We must clearly understand the impact of construction on the design intent....for instance if we design a moment connection and it gets built with only part of the connection in place for moment transfer, then the stresses normally transferred as a moment will get re-distributed somewhere else (and perhaps where they cannot be accommodated....that's one of the things that causes structural failures!)

 

[J1D]

I agree with Ron on checking the AS-BUILT. For engineering design, I would tackle the support fixity in following ways:

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.