waqar saleem

masnory wall on RCC footing

14 posts in this topic

Salam

Dear friends

i need guidance on the foundation of masonry structure (2 storey residential with partial basement) on week soils, (BC 0.75tsf), which foundation will perform better, i) an strip RCC footing with an upstand/down beam under the wall. ii) strip RCC footing without beam.Share your experiences and any other good suggestions.

 

Regards 

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On 2016-12-26 at 2:50 AM, waqar saleem said:

i) an strip RCC footing with an upstand/down beam under the wall. ii) strip RCC footing without beam.

My first question is that why are you asking this question? What is your actual concern.. (shear, moment, settlement etc?). Also clarify if the beam would be of same width as that of wall.

In my opinion, considering beam is of same width as that of wall, there would be no significant difference. 

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Thanks

my question is regarding foundation on weak soil, i came across the practice of providing beams under masonry walls, beams are of same width but depth greater than footing(like upstand beam in slabs). beam acts as inverted T-beam. i am asking the difference of soil structure interaction of foundation with beam and without beam.

 

Regards

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My opinion is that, beam foundation is mostly suited when loads coming from super structure are excessive and causing punching shear in footing. So instead of increaisg depth of whole footing, we provide tapered footing or we go for beam footing. But since in case of wall, there is no case of punching shear. And only one way shear exists. Which is nearly same either you provide beam or not. Again if u r having excessive one way shear u can again go for beam footing or tapered footing. again if u think tapering will cost more go for localized thickneing by providing beam. More over, providing beam also causes increment in rigitiy of footing. I will personally provide strap footing. 

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For design of beam footing, either you cand consult PRC 1 by Z A SIDDIQUE or search the forum. There are other discussions also.

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If you are worring about settlement of foundation, i think it can be checked in safe. If beam without bem satisfies, strap footing must be enough. 

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On 2016-12-29 at 3:20 AM, waqar saleem said:

Thanks

my question is regarding foundation on weak soil, i came across the practice of providing beams under masonry walls, beams are of same width but depth greater than footing(like upstand beam in slabs). beam acts as inverted T-beam. i am asking the difference of soil structure interaction of foundation with beam and without beam.

 

Regards

I can't see any difference.

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i guess the one with beam will perform better than the simple foundation.

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A wall footing with an RC beam (of height greater than the thickness of footing slab) will have a greater stiffness than the one without beam. As such, it will have more capacity to resist longitudinal bending of wall / footing caused by any concentrated loads, as well as in spanning over some loose pockets existing within the soil beneath.

Regards. 

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5 hours ago, waqar saleem said:

i guess the one with beam will perform better than the simple foundation.

Depends on loading nd settlement. If loads are high, go for beam. Difference i have mentioned above. It is you who will have final judgement. 

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10 hours ago, EngrUzair said:

A wall footing with an RC beam (of height greater than the thickness of footing slab) will have a greater stiffness than the one without beam. As such, it will have more capacity to resist longitudinal bending of wall / footing caused by any concentrated loads, as well as in spanning over some loose pockets existing within the soil beneath.

Regards. 

I hold the same opinion on this topic.

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19 hours ago, EngrUzair said:

A wall footing with an RC beam (of height greater than the thickness of footing slab) will have a greater stiffness than the one without beam. As such, it will have more capacity to resist longitudinal bending of wall / footing caused by any concentrated loads, as well as in spanning over some loose pockets existing within the soil beneath.

Regards. 

 

8 hours ago, BAZ said:

I hold the same opinion on this topic.

 

Valid points but any requirements for shear and moment demand should be met by "increasing the thickness of foundation" rather than relying on a beam to do so. The reason being that foundation (in this case strip foundation) reinforcement and formwork requirements are very simple and less labour intensive. Introducing a beam would make it complicated and more costly and in some cases an overkill. Also beam foundation will have no significant affect on settlement as I believe settlement is a function foundation width and bearing pressure, which in both cases would be similar. 

 

Edited by Ayesha

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2 hours ago, Ayesha said:

Introducing a beam would make it complicated and more costly and in some cases an overkill.

Agreed, in general. However, there are situations where provision of an RC beam and even RC wall becomes necessary.

My previous reply was with reference to the general case of two wall footings of same slab size (both in plan and in thickness), but one of them with an RC beam of height more than the thickness of footing slab.

In practice however the type of footing used will vary with the site situation. For example, in case of an ordinary two storey masonry wall building without basement, simple strip footing (of a uniform thickness) should be ok in most of the strong soils with uniform strata. On the other hand, strip (or even a raft) footing with beam(s) may however be required to avoid differential settlement for sites with scattered loose subsoil pockets. 

However, for the specific cases of buildings with a basement, either in weak soils or at locations where the subsoil water level is high, provision of even an external RC wall (for the purpose of retaining earth, or prevention of moisture inside the building etc.) becomes desirable.  The problem highlighted in the Original Post also falls in this latter category.

Regards.

 

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10 hours ago, Ayesha said:

any requirements for shear and moment demand should be met by "increasing the thickness of foundation" rather than relying on a beam to do so.

If beam is wide it reduces one way shear force by reducing cantilever part. but providing beam or not, is purely dependant on situation and loading. 

10 hours ago, Ayesha said:

Also beam foundation will have no significant affect on settlement as I believe settlement is a function foundation width and bearing pressure, which in both cases would be similar. 

I believe it is mostly differential settlement that happens and when one part of wall exerts more load and soil loses its strength and settles, then the beam provided will behave as bridge between this settled soil and hence will not allow wall to settle even if soil has settled( as in loose pockets). 

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