Lateral Inertia from Beams to Soil

[lisyuse]

In lateral movements. Inertia is created in the roof or floor and this is transferred to beams.. to columns.. to foundation.. and to soil.. but what if the soil is stiff like rock and you have a more than average sized foundation for complete fixed rotational restraint of the columns. How does the stiff soil/rock and big foundation interact with the inertia from upper that must be transferred to the soil/rock?

 

[lisyuse]

so let me rephrase the question

in page 7 of

http://nehrp.gov/pdf/nistgcr8-917-1.pdf

"Base restraint can have a significant effect on the behavior of a moment frame. ASCE 7 - 12.7.1 (Foundation Modeling) states “for purposes of determining seismic loads, it is permitted to consider the structure to be fixed at the base."

Question. If the structure is fixed at the base (fully moment connected becoming rigid).. there is zero rotations at the base.. so likewise there is zero rotations at the beam-column joints above.. so why do you still have to determine seismic loads when rotations in all the joints have been suppressed to zero.. unless it is referring to the columns buckling?

 

[jimstructures]

Having a Fixed Base column does not prevent rotations at the beam-column joints above the fixed base. It might reduce them but it doesn't eliminate those rotations.

Jim

 

[structSU10]

The beam-column joint itself is considered to be rigid, but it would undergo something like a rigid-body rotation as the beam and column element have flexibility. At least in a simplified sense. The joint itself is rigid, but the members that create the moment frame a fairly flexible. when flexible members frame into rigid boundary conditions, it improves their apparent rigidity and can reduce their deformations.

 

[IDS]

Lisyuse - Try sketching a simple 2D frame structure, then sketch the shape you think it would deflect to if a horizontal force was applied at the top. Add shear force and bending moment diagrams. Then analyse it in a frame analysis program and check your sketches. If they don't match, work out why they don't.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[lisyuse]

Having a Fixed Base column does not prevent rotations at the beam-column joints above the fixed base. It might reduce them but it doesn't eliminate those rotations.

Jim

If you make the columns very big like 1 meter diameter and add so many rebars like 50 pcs of 25mm grade 60 rebars.. it would still deflect? Or become completely rigid. When this is fully moment connected fixed to the foundation and there is lateral movement, the column would no longer deflect and the above beams and beam-column joint may no longer rotate but just move or translate along with the foundation. Not?

 

[Ron]

NO! The foundation should not be connected to the rock! How do get from "I would not pin the foundation to the rock" to "So you are saying the foundation must be *fixed* to the rock"? That's not what I'm saying at all!!

 

[lisyuse]

NO! The foundation should not be connected to the rock! How do get from "I would not pin the foundation to the rock" to "So you are saying the foundation must be *fixed* to the rock"? That's not what I'm saying at all!!

Ok. It's neigher pinned not fixed.. meaning the foundation is just *put* on top of the rock. You are worried any movement of the rock can transfer to the foundation.. but the foundation is mat foundation.. so any movement of the rock should transfer to it due to the coefficient of friction making them move together as one and not slide.. are you referring to smaller foundation where you prefer it to just slide against the rock?

 

[BAretired]

This thread seems to be moving in many directions. It has not previously been established that the columns are very big with large amounts of reinforcement but even if that were the case, the column would still deflect under lateral load. It can never be completely rigid.

A column fixed against rotation at Point A (the base) and also at point B, the elevation of the upper floor beams will bend in double curvature ("S" shape) under lateral load and will develop equal moments at points A and B with zero moment at mid-story height. In order for that to occur, the beams at Level B would need to have infinite stiffness which is not going to be possible, so the column at Level B cannot have zero rotation under lateral load.

BA

 

[lisyuse]

1. So in general the advantage of column base and foundation rotational restrain is it will take greater seismic load to form plastic hinges in the beam ends?

2. Can shear wall and braced frames make the beam-column joint completely rigid or will it still rotate? how many percentage compared to fully rotationally restrained column base and foundation base (say against stiff rock).

 

[BAretired]

1. That would certainly be one of the advantages. Another is that the structure will deflect less when fixed at the base, thus reducing secondary effects.

2. Shear walls and braced frames can reduce rotation of the beam-column joint but nothing in structural theory is "completely rigid". I would not hazard a guess at percentages of one system versus another. Perhaps someone else would like to take a crack at that question. I come from an area in Canada which does not experience significant seismic events, so I am probably not the best person to ask. I have heard, however that flexible structures behave best on rock or stiff soil whereas stiff structures tend to do better on soft soils. I haven't researched it but it sounds reasonable to me. Again, some of the more experienced members may wish to comment further on seismic considerations.

BA

 

[wilberz]

I think you are "over thinking" this. Structural analysis is more simple than that. Inertial movement is small and generally absorbed by the stiffness of the framing. If your foundation is sized as noted, the lateral confinement of the soil-structure interaction is moot.

I think you are "under thinking" this. Structural analysis is more complicated than that. Inertial movement is big and generally absorbed by the ductility of the frames and not so much the stiffness. This is owing to the fact that stiffness can attract more seismic forces. Could traverse ties in the columns be able to handle these seismic forces? How do other fellow engineers here think about this issues?

 

[Ron]

wilberz...you are correct with regard to seismic action; however, the OP was discussing overturning which is moreso associated with wind. With the exception of high rise structures, there is usually not enough inertial movement transfer to the foundation level to be of concern. Two different conditions, two different approaches.....and yes, seismic considerations are more complex than wind issues.