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
RE: Lateral Inertia from Beams to Soil
RE: Lateral Inertia from Beams to Soil
BA
RE: Lateral Inertia from Beams to Soil
RE: Lateral Inertia from Beams to Soil
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
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
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
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?
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
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
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
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
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
BA
RE: Lateral Inertia from Beams to Soil
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
BA
RE: Lateral Inertia from Beams to Soil
RE: Lateral Inertia from Beams to Soil
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
BA
RE: Lateral Inertia from Beams to Soil
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
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
Jim
RE: Lateral Inertia from Beams to Soil
RE: Lateral Inertia from Beams to Soil
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Lateral Inertia from Beams to Soil
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
RE: Lateral Inertia from Beams to Soil
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
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
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
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
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