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Lateral Inertia from Beams to Soil

Lateral Inertia from Beams to Soil

Lateral Inertia from Beams to Soil

(OP)

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?

RE: Lateral Inertia from Beams to Soil

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.

RE: Lateral Inertia from Beams to Soil

(OP)
It is said that when you designed the column to foundation connection to be fixed (and not pinned).. the foundation must transfer the moment (bigger values) to the soil.. but what if the soil is stiff.. what does it mean to transfer the moment from the foundation to soil especially if the soil is stiff/rock and can't bend?

RE: Lateral Inertia from Beams to Soil

It means the foundation is fixed against rotation.

BA

RE: Lateral Inertia from Beams to Soil

(OP)
BAretired.. it means if the soil is soft.. you need to make the foundation big enough such that the entire foundation won't rotate against the soil? meaning about 4 times bigger than normal. Because when the column to foundation is pinned.. it is the column that would rotate.. but if the column to foundation is fully fixed.. it is the foundation that would rotate against the soil.. and to prevent rotation in soft soil.. what does the different code says about oversizing the foundation so it doesn't rotate.. what is the formula for this in addition to the normal bearing pressure formulas?

RE: Lateral Inertia from Beams to Soil

You are changing the question. You asked "what does it mean to transfer the moment from the foundation to soil especially if the soil is stiff/rock and can't bend?" If the soil is stiff rock, the foundation cannot rotate significantly and is approaching full fixity.

In soft soil, the foundation can rotate and settle which means it is not fixed against rotation or translation. The amount of rotation and translation depends on the properties of the soil. You could consider the foundation to be a rigid body in an elastic medium and determine a rotational and vertical spring stiffness to account for the elastic deformation of the soil, but it is not likely to provide a very precise answer because soil is not a uniformly elastic, isotropic material.

BA

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

In soft soil, the foundation can rotate and settle which means it is not fixed against rotation or translation. The amount of rotation and translation depends on the properties of the soil. You could consider the foundation to be a rigid body in an elastic medium and determine a rotational and vertical spring stiffness to account for the elastic deformation of the soil, but it is not likely to provide a very precise answer because soil is not a uniformly elastic, isotropic material.

So in areas with soft soil. Even if you can make fixed (rotational restrained) column-foundation (rather than pinned) connection. It doesn't help much because the moment is transferred to the foundation which can rotate aginst the soft soil? So fully fixed rotationally restrained column-foundation connection is only useful for stiff soil/rock?

RE: Lateral Inertia from Beams to Soil

Quote (lisyuse)

what does it mean to transfer the moment from the foundation to soil especially if the soil is stiff/rock and can't bend?

It means if the soil (rock) does not "bend" under the footing rotation, then the footing can only overturn (or fail in bending) under the induced moment. If you have designed with sufficient mass to prevent overturning and sufficient flexural reinforcement, don't worry about it.

You could pin the footing to the rock, but I wouldn't.

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

It means if the soil (rock) does not "bend" under the footing rotation, then the footing can only overturn (or fail in bending) under the induced moment. If you have designed with sufficient mass to prevent overturning and sufficient flexural reinforcement, don't worry about it.

Ok, I was tying it to physics.. Inertia has energy and momentum, it has to go elsewhere.. so it either goes to the ground "bending" the soil or every fabric and molecules of the rebars have to compress and take in the energy.. in other words.. act elastically..

Has anyone designed seismic building where the seismic energy has to dissipate elastically (without any ductile plastic moments developing). How strong is this structure?

Quote:

You could pin the footing to the rock, but I wouldn't.

Why won't you? Even if it's pinned, it can't rotate against the rock.. so why hesitate pinning it? Or what are you trying to say?

RE: Lateral Inertia from Beams to Soil

As I said before, you are way overthinking this issue!

I would not pin the foundation to rock below because this causes additional restraint to the foundation thus increasing the potential for cracking the foundation. Further, any movement of the rock will be transferred to the foundation.

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

As I said before, you are way overthinking this issue!

I would not pin the foundation to rock below because this causes additional restraint to the foundation thus increasing the potential for cracking the foundation. Further, any movement of the rock will be transferred to the foundation.

So you are saying the foundation must be *fixed* to the rock. How do you fix it (versus just pinning it)?

RE: Lateral Inertia from Beams to Soil

Quote (OP)

So in areas with soft soil. Even if you can make fixed (rotational restrained) column-foundation (rather than pinned) connection. It doesn't help much because the moment is transferred to the foundation which can rotate against the soft soil? So fully fixed rotationally restrained column-foundation connection is only useful for stiff soil/rock?

Fixing the column to the footing helps in any type of soil because it creates a moment at the base of the column thereby reducing moments at other locations. The base moment will be somewhere between fixed and hinged. Its magnitude depends on the rotational resistance provided by the soil.

BA

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

Fixing the column to the footing helps in any type of soil because it creates a moment at the base of the column thereby reducing moments at other locations. The base moment will be somewhere between fixed and hinged. Its magnitude depends on the rotational resistance provided by the soil.

When you brace the frames.. there is less moment demands on the beams. Likewise if you fixed the column-foundation, there is less moment demands on the beams...

But it seems many structural engineers would rather design better beams to withstands seismic moments than designing better column-foundation rotational restrained.. why? Is it really cheaper to make better beams than better foundations in the experience of those very long already in the industry here.

RE: Lateral Inertia from Beams to Soil

Too many variables to come up with a definitive answer to the last question. Reliance on soft soils to provide rotational restraint during seismic activity may be a questionable strategy.

BA

RE: Lateral Inertia from Beams to Soil

(OP)

If you can't make shear walls or braced frames and rely on column foundation rotational restraint (*assuming* you have a stiff soil/rock).. how much can it equal the first two in effectivity?

If you have stiff soil and you don't want to use shear walls or braced frames. How much can you rely solely on column-foundation rotational restraint and oversized foundation (again on stiff soil/rock.. this time we are not talking about soft soil I know).

RE: Lateral Inertia from Beams to Soil

If the foundation is a pile or caisson drilled into stiff soil or rock, I would think that a certain amount of rotational restraint could be relied upon. If the foundation is a shallow footing bearing on stiff soil or rock but not well anchored in the rock, rotational restraint is minimal, particularly in the case of moments resulting from a seismic event.

BA

RE: Lateral Inertia from Beams to Soil

(OP)
If rotational restraint can really be RELIED upon.. like using mat foundation on stiff rock. Then it can be an alternative to shear walls and braced frames? Because shear walls need to be symmetrical or torsion can be introduced where the other parts rotate against the stiffer unsymmetric shear walls.. and braced frames is difficult for purely concrete frames.

RE: Lateral Inertia from Beams to Soil

(OP)

Ok. I'm analyzing this material about special moment frames.. http://nehrp.gov/pdf/nistgcr8-917-1.pdf

page 7 mentions:

"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.. there is zero moments at the base.. so likewise there is zero moments at the beam-column joints above.. so why do you still have to determine seismic loads when moments in all the joints have been suppressed to zero.. unless it is referring to the columns buckling?

Likewise in the same page.. it is mentioned "If the drift of the structure exceeds acceptable limits, then rotational restraint can be increased at the foundation by a
variety of methods, as illustrated in Figure 4-1 (b), (c), and (d)."

Question. Why not suppressed all drifts altogether by fully rotationally restraining the columns bases (fully fixed)?

RE: Lateral Inertia from Beams to Soil

Question by lisyuse (OP). If the structure is fixed at the base.. there is zero moments at the base..really?...think again! so likewise there is zero moments at the beam-column joints above.. so why do you still have to determine seismic loads when moments in all the joints have been suppressed to zero.. unless it is referring to the columns buckling?

BA

RE: Lateral Inertia from Beams to Soil

(OP)

I know the base is moment connected.. what I meant was zero rotation because it is fixed.. so if the column has zero rotation at the base.. likewise it has zero rotation above at the beam column joint so why worry about seismic forces in the beams? the beams cant rock back and forth or sway from side to side...

RE: Lateral Inertia from Beams to Soil

(OP)
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?

RE: Lateral Inertia from Beams to Soil

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

RE: Lateral Inertia from Beams to Soil

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.

RE: Lateral Inertia from Beams to Soil

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/

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

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?

RE: Lateral Inertia from Beams to Soil

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!!

RE: Lateral Inertia from Beams to Soil

(OP)

Quote:

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?

RE: Lateral Inertia from Beams to Soil

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

RE: Lateral Inertia from Beams to Soil

(OP)

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).

RE: Lateral Inertia from Beams to Soil

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

RE: Lateral Inertia from Beams to Soil

Quote:

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?

RE: Lateral Inertia from Beams to Soil

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.

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