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Modelling of RC foundation beams

Modelling of RC foundation beams

Modelling of RC foundation beams

(OP)
Hello everyone,

I have designed the superstructure of an RC frame 2-storey house using fully fixed supports at the base of the columns.
(design is according to EC2 and EC8)

Now i want to design the foundations beams of the structure. In order to correctly model the foundation beams i need
to remove the fully fixed supports at the base of the columns right?

And then i will need to add an elastic support along the length of the beams, right?

Is my thinking correct?

RE: Modelling of RC foundation beams

Quote:

Now i want to design the foundations beams of the structure. In order to correctly model the foundation beams i need
to remove the fully fixed supports at the base of the columns right?

Depends on the detail. Hard to say without it.

Quote:

And then i will need to add an elastic support along the length of the beams, right?

Yes....and with a considered possibility of no support if the span is stiff enough and the (long-term) settlement is high enough.

RE: Modelling of RC foundation beams

(OP)
Ok let me add more information

I have created two models 1) one with fully fixed supports at the base of the columns and 2) one with out the fixed supports at the base of the columns.

Both models have the same foundation beams with the same elastic supports.


The analysis results show that the foundation beams on the model with out the fully fixed supports have considerably increased bending moments and shear forces


Please see results below:

foundation beams with fully fixed supports: bending moment = 8KNm, shear force = 17kN

foundation beams with OUT fully fixed supports: bending moment = 180kNm, shear force = 172 kN

As you can see there is considerable increase in both bending and shear, is this realistic?

RE: Modelling of RC foundation beams

Quote:

The analysis results show that the foundation beams on the model with out the fully fixed supports have considerably increased bending moments and shear forces


Not surprising. But it's doubtful that base will develop full fixity: your foundation (assuming we are talking about a footing here) will rotate a bit......and that moment will change. Again: it's hard to advise without a detail. I've seen grade beams come in several feet above the top of the footing.....and in other cases it's sitting right on the footing. These 2 scenarios will obviously have differences in rotational stiffness.

RE: Modelling of RC foundation beams

(OP)
Hi,

This is the type of foundation i am using, its a monolithic slab (raft foundation) 45cm thick, with foundation beams 45cm height fully embedded within the foundation slab, interconnecting all columns at the base.

Please see link for pictures and more detail

http://www.eng-tips.com/viewthread.cfm?qid=429281





RE: Modelling of RC foundation beams

Pretty stout foundation for a house.

Ok, I had envisioned a footing at the column bases but it appears we are talking just grade beams alone. Ergo, you would probably just model a bunch of grade beams (with a elastic base) spanning between the columns (with a slab).

RE: Modelling of RC foundation beams

(OP)
Ok, so you suggest I remove all fully fixed supports at the base of the columns, and add an elastic support to the foundation beams to simulate ground stiffness?




Initially i want to design the beams with out considering the monolithic slab just to check the reinforcement required.
Once the beams are designed i will also take into consideration the slab reinforcement and probably change the shape of the beam to inverse L-shaped and T-shaped.

This is a plan view of the foundation beams,

https://res.cloudinary.com/engineering-com/image/upload/v1503952535/tips/Substructure_-_Foundations_rnp7dz.pdf

click the link above for a larger picture



RE: Modelling of RC foundation beams

Quote:

Ok, so you suggest I remove all fully fixed supports at the base of the columns, and add an elastic support to the foundation beams to simulate ground stiffness?

Right. Your foundation model (not considering the slab) would be a bunch of (meshed) stick elements that represent the grade beams. The support for the grade beams would be vertical springs only. If you include lateral loads in your model.....you could just have some horizontal/lateral restraints at the column bases. (And check with hand calculations that you can mobilize enough frictional/passive resistance at those locations to make it valid.)

But all the rotational restraint at your column bases should be from your grade beams.....not any "fixed" support node.

RE: Modelling of RC foundation beams

(OP)
Actually I am designing for earthquake resistance according to Eurocode 8.

When you say horizontal/lateral restraints you mean a pin support?

RE: Modelling of RC foundation beams

Quote:

When you say horizontal/lateral restraints you mean a pin support?

No because that would mean infinitely stiff in the vertical direction.....and you don't want that. Basically designate the column base node as a support, but release all rotational degrees of freedom and have the vertical be a spring with the appropriate stiffness. (In STADD, that's done with the FIXED BUT command for supports.)

As a result, you will come out with vertical and horizontal reactions.

RE: Modelling of RC foundation beams

(OP)
Thanks a lot for your time you have been very helpful. I have one more question.

Since the foundation beams are fully embedded with in the slab, its reasonable to assume that the slab will also provide some stiffness to the foundation beam,
how should i calculate the effective flange width of the foundation beam? i didnt find this in any text.

Also what about the thickness of the flange? is it going to be the full thickness of the slab?

RE: Modelling of RC foundation beams

Quote:

Since the foundation beams are fully embedded with in the slab, its reasonable to assume that the slab will also provide some stiffness to the foundation beam, how should i calculate the effective flange width of the foundation beam? i didnt find this in any text.

.......

Also what about the thickness of the flange? is it going to be the full thickness of the slab?


You are under the Eurocode so I'm not sure what to tell you there. ACI 318-11 says this:

---------------
8.12— T-beam construction

8.12.1 — In T-beam construction, the flange and web
shall be built integrally or otherwise effectively bonded
together.

8.12.2 — Width of slab effective as a T-beam flange
shall not exceed one-quarter of the span length of the
beam, and the effective overhanging flange width on
each side of the web shall not exceed:

(a) Eight times the slab thickness; and
(b) One-half the clear distance to the next web.

8.12.3 — For beams with a slab on one side only, the
effective overhanging flange width shall not exceed:

(a) One-twelfth the span length of the beam;
(b) Six times the slab thickness; and
(c) One-half the clear distance to the next web.

8.12.4 — Isolated beams, in which the T-shape is used
to provide a flange for additional compression area,
shall have a flange thickness not less than one-half the
width of web and an effective flange width not more
than four times the width of web.

8.12.5 — Where primary flexural reinforcement in a
slab that is considered as a T-beam flange (excluding
joist construction) is parallel to the beam, reinforcement
perpendicular to the beam shall be provided in
the top of the slab in accordance with the following:

8.12.5.1 — Transverse reinforcement shall be
designed to carry the factored load on the overhanging
slab width assumed to act as a cantilever. For isolated
beams, the full width of overhanging flange shall be
considered. For other T-beams, only the effective overhanging
slab width need be considered.

8.12.5.2 — Transverse reinforcement shall be
spaced not farther apart than five times the slab thickness,
nor farther apart than 18 in.
-------------------------



RE: Modelling of RC foundation beams

(OP)
I guess this is for ordinary beams and slabs spanning between columns, I know where I can find that in Eurocode but cant find anything regarding foundation beams and slabs.
Maybe there is something in ACI specific to foundation beams?

Probably its something similar but would be nice to find something specific.

RE: Modelling of RC foundation beams

Quote:

I have designed the superstructure of an RC frame 2-storey house using fully fixed supports at the base of the columns.(design is according to EC2 and EC8) Now i want to design the foundations beams of the structure.

If the superstructure design is finished and you're not going to revisit it, you should design the foundation beams on the basis that the applied loads are the support reactions from the superstructure analysis. The analysis would be just the foundation with applied loads and would not include the superstructure.

Quote:

In order to correctly model the foundation beams i need to remove the fully fixed supports at the base of the columns right?

This will change the load distribution throughout the structure including the superstructure. If you're going to design the foundation beams for the stresses from this analysis, you should re-check the superstructure design for the new stresses also.

RE: Modelling of RC foundation beams

(OP)

Quote (steveh49)

This will change the load distribution throughout the structure including the superstructure. If you're going to design the foundation beams for the stresses from this analysis, you should re-check the superstructure design for the new stresses also.

I will not actually remove the supports complete, i will modify them by releasing all rotational degrees of freedom and have the vertical be a spring with an appropriate stiffness

RE: Modelling of RC foundation beams

(OP)
As i mentioned above, in order to model the foundations on elastic soil I have modified all fully fixed supports at the column bases
-> By releasing all rotations and assigning a stiffness to the Z+ direction.

As expected all of my dead loads and live loads have now changed from linear static to nonlinear static analysis.
Now when i try to design the foundations the design only takes into account the seismic load combinations, the ULS and SLS combinations are not considered.

Any suggestions why?


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