Large column-point load on top of a basement wall?
Large column-point load on top of a basement wall?
(OP)
My dear friends,
This one always drives me crazy. Perhaps it is because I have no great feel for this one:
You have 4 R.C walls immersed in soil, walls 1ft thick. At the corners or middle of one of these walls you have a steel column pushing in some 40tons.
Which one is more correct?
1) Take an arbitrary strip, say 2ft wide and analyze it for active pressure as well as the 40ton, design it as a stocky column(no second order effects) and tie the strip just like you would a column. Honestly I have no number to attach to my ties. Some minimum will do kind of rationality.
2) Carry out finite element analysis with any standard software, get membrane forces(pressure) and simply provide vertical reinforcement as you would with a wall. You have your min. reinforcement as that of any other R.C wall. In this case I dont tie.
I do both and I am not very happy with so much a difference.
Any discussion will do.
Thanks.
IJR
This one always drives me crazy. Perhaps it is because I have no great feel for this one:
You have 4 R.C walls immersed in soil, walls 1ft thick. At the corners or middle of one of these walls you have a steel column pushing in some 40tons.
Which one is more correct?
1) Take an arbitrary strip, say 2ft wide and analyze it for active pressure as well as the 40ton, design it as a stocky column(no second order effects) and tie the strip just like you would a column. Honestly I have no number to attach to my ties. Some minimum will do kind of rationality.
2) Carry out finite element analysis with any standard software, get membrane forces(pressure) and simply provide vertical reinforcement as you would with a wall. You have your min. reinforcement as that of any other R.C wall. In this case I dont tie.
I do both and I am not very happy with so much a difference.
Any discussion will do.
Thanks.
IJR





RE: Large column-point load on top of a basement wall?
I'm sorry this doesn't address the reinforcing issue. Also, check out Bowles' Foundation Engineering. Seems to me I've seen this application somewhere before in a text. I just can't place it right now. I'll be on the lookout for it though.
RE: Large column-point load on top of a basement wall?
If you come out with anything extra, please let us know.
RE: Large column-point load on top of a basement wall?
Specifically, they require a concentrated load to be applied over a width of wall no greater than 4 times the wall thickness (plus width of bearing)....14.2.4.
Section 14.3.6 states that, "Vertical reinf. need not be enclosed by lateral ties if vertical reinforcement area is not greater than 0.01 times gross area...."
Section 14.4 requires walls to also be designed per numerous other sections in chapter 10 (column design).
With this, unless your wall is somewhat lightly loaded, you must design it as you would a column and use your option 1.
Qshake is right, the FE analysis would be a lot of work.
RE: Large column-point load on top of a basement wall?
RE: Large column-point load on top of a basement wall?
Are your basement walls topped up with a slab? If not, than its a cantilevering wall!
Fine, both approaches are correct. In both instances the vertical load on the wall creates a clamping effect. If this force can resist the overturning effect due to active forces by more than 4 implies u have an ideal prop.
Note the column load is dispersed at 45 degrees from the point of application. U need to check the maximum compressive stress on wall against allowed.
Riz
RE: Large column-point load on top of a basement wall?
JAE above mentioned code-specific solutions. I am going into that now.
As for your response, seems logical too. But me, used to detailing steel, tend to somehow treat dispersion of point loads the way we treat them in steel beams, I mean somewhere there is a critical section with a code-specified area, and we check bearing or buckling there.
What about your dispersion concept, where should I check and how?. In case you have time.
RE: Large column-point load on top of a basement wall?
That stress distribution is significantly different than what you will get using the simplified column approach.
RE: Large column-point load on top of a basement wall?
Million thanks. You got a way to look at and make things simpler and clearer.
IJR
RE: Large column-point load on top of a basement wall?
I have checked ACI 318 clause 14.2.4 which mentions that the effective strip can not exceed bearing width + 4 TIMES THE WALL THICKNESS.
When applied to a point load at some corner where 2 walls intersect, do you use 2 TIMES WALL THICKNESS ON EACH DIRECTION. I mean at location X in plan.
-----------X
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RE: Large column-point load on top of a basement wall?
RE: Large column-point load on top of a basement wall?
Thanks.
RE: Large column-point load on top of a basement wall?
45 Degrees either side of the column centre line. Shear concept!
Authority to check: Foundation Design by A. Hodgkinson, 1986, Architectural Press, London.
Riz
RE: Large column-point load on top of a basement wall?
Now, the point load is going to disperse over a distance say 2 m below covering a strip 4m, right?. I will have a beam-column section there of 400cm*30cm, if my wall is 1ft wide. A metre above I will have a lesser section, well with a lesser moment coming from lateral pressure.
Lets discuss your idea: How do I go finding section from which I can use Moment and axial force (M,N) to help me detail the wall as a column. Are you against treating the wall as a column?. Are you suggesting direct use of membrane forces (I mean force/m kind of approach)?
Thanks for consideration. Be back.
IJR
RE: Large column-point load on top of a basement wall?
Now, the point load is going to disperse over a distance say 2 m below covering a strip 4m, right?. I will have a beam-column section there of 400cm*30cm, if my wall is 1ft wide. A metre above I will have a lesser section, well with a lesser moment coming from lateral pressure.
Lets discuss your idea: How do I go finding section from which I can use Moment and axial force (M,N) to help me detail the wall as a column. Are you against treating the wall as a column?. Are you suggesting direct use of membrane forces (I mean force/m kind of approach)?
Thanks for consideration. Be back.
IJR
RE: Large column-point load on top of a basement wall?
At your corner condition, I would approach the design of the wall "column" as 2T in one direction and 2T in the other...sort of an "L" column - for the column design. This would be an irregular column that could be designed by hand (difficult) or using software like PCACOL (IRRCOL module).
Now, for the footing design, I would agree with the other comments that you can distribute the load over a 45 degree (or 30 degree if your conservative) distance.
RE: Large column-point load on top of a basement wall?
Your comment also makes Riz's more clear to me.
Also special thanks to Ron, Riz and Qshake. It has been a great discussion.
IJR