effect of support settlement

[bazflexure]

Will anybody comment on the effect of support settlement of a frame column.
One effect will be of change in stresses.
Will it increase reaction of that particular support.If yes then how?

 

[racookpe1978]

Well, usually, when the support under a column or frame settles the column goes down as well: 8<)

Two ways of looking at it seriously though:

1) The column was loaded up with some weight, and that weight was adequate and correctly calculated for the column and frames, but the concrete pad under the column couldn't hold the load.

Dirt and fill washed out under the concrete => the pad is going to keep going down (pulling the column with it) until your building fails;

dirt wasn't compacted enough under the concrete before the concrete was poured and so it will keep compresing until the dirt and fill is squished enough to stop moving => If so, then maybe your column will stop moving down before your building fails;

the concrete isn't the right size, or wasn't poured or mixed correctly, or the column baseplate is too small or missing and the column is driving "through" the concrete. (Alt: the concrete is driving down through the mud and fill under under it.) => Your column will continue driving the concrete down and the building will fail.

2) Your concrete and fill are good enough (neither is moving) but the column is not sized right for the load that is actually bearing on it. Usually, this would be a buckling though, not a simple "settling" or lowering of the column.

Immediately get your people safe, get the site safe for future work.

Then remove weight from the column, change your design, reinforce the column and replace it, (don't try to simply straighten it - it will fail again); fix and replace the coumn joints and connections, examine the adjacent columns and frames and see what needs to be replaced.

3) Hire a good lawyer and a big banker - you will need both.

 

[BAretired]

It seems intuitive that support settlement of one frame column in a statically indeterminate frame will tend to decrease the reaction of that column, not increase it.

If the frame is statically determinate, then settlement of one column has negligible effect on column reactions.

BA

 

[cntw1953]

For structurally indeterminate structure, support settlement will induce add'l internal moments on members, thus affecting the support reactions (either increase or decrease). This topic is covered in elemtary structural analysis textbooks.

 

[racookpe1978]

Well, that's what I'm trying to point out:

The column is settling (moving down) becuase it (or its pad and the slab) cannot stop (resist) what ever load is acting on it. It will do one of three things:

(1) Keep moving down until the load is resisted by sharing (increasing the load) on adjacent horizontal and vertical members until they pick up enough so the member and pad can handle the load; or

(2) stop moving vertically when the resistance under the pad changes enough so the pad can handle the load. This stops the pad from settling and allows the column begin resisting the load.

(3) Break the building by excessive settlement, cracking, and deformation of the frame.

 

[BAretired]

As I have said earlier, settlement of a support tends to reduce the reaction at that support. Does anyone have anything which would suggest otherwise?

BA

 

[SEwannaB]

Yep, BA that's all that happens given what we know. If the soils are springs and not able to support the load, the foundation will move downward, relieving load on the column. If moment framing, redistribution thru the connecting members will increase their applied load and decrease the column load. And if non-moment resistant framing, little change in column load.

racook, you're right but just more than is asked. And definitely right about the legal.

 

[Firecodeman]

I am a licensed professional engineer and not familiar with structural mat foundations. My questions are basic in nature as follows. If one was to install a mat foundation for a 2-story plus a basement single family house approxiamate size 40 feet by 100 feet is there a minimum required height that the mount foundation should be from the water level below the house? The house was built in an X flood zone which is now AE flood zone with the top of basement slab being 6.45 above NAD of 1988. Presently the water level appears to be 2.5 feet below the mat basement slab but also appears to vary from 2 to 4 feet or more depending on the rain fall and local water table. Lastly if the house has been built for three years and there are only minimal cracks in the mat basement slab and it is still level, what is the likelyhood of it settling more over time?