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?

 

[Ron]

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.

 

[lisyuse]

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?

 

[BAretired]

It means the foundation is fixed against rotation.

BA

 

[lisyuse]

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?

 

[BAretired]

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

 

[lisyuse]

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?

 

[Ron]

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.

 

[lisyuse]

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?

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?

 

[Ron]

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.

 

[lisyuse]

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)?

 

[BAretired]

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

 

[lisyuse]

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.

 

[BAretired]

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

 

[lisyuse]

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).

 

[BAretired]

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

 

[lisyuse]

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.

 

[lisyuse]

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)?

 

[BAretired]

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

 

[lisyuse]

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...