Beam on top of wall

[alphaxy]

Fellow engineers,

I have a beam on top of wall (as shown in attachment). What should be the methods of checking this beam and wall? There is a column on top of it at every 5 meters and a transverse wall also at every 5 meters. The beam is on top of wall throughout its wall length.

I was task to check if the size and its reinforcement are still adequate using the provisions of ACI-05.

Pls. advise.

 

[msquared48]

From the placement of the reinforcing in the "beam", it looks like it is either a drag link, or, more likely, lateral support for the top of the wall.

Mike McCann
MMC Engineering

 

[JAE]

It depends on the type, magnitude and direction of loading on the wall. I would assume the columns would at least transfer vertical loads into the beam and thus into the wall. Is there liquid lateral forces on the wall? Wind/seismic lateral forces? You have to analyze something before you can design-check it.

 

[alphaxy]

msquared48 & JAE,

Thank you very much for your response.

The walls are loaded by water & soil pressures. The detail I've shown to you is similar to the transverse support @ every 5.6 meters and columns planted on top of beam.

The loads coming from the column are from the structure above. These walls with beam on top is a basin carrying water.

Can you tell me how should be the beam analyzed? I was thinking of checking the out of plane bending, shear in plane and out of plane and torsion, under doubly reinforced beam condition. Is this correct? Or will it behave like a column?

 

[hokie66]

From your description, this is a capping beam whose main function is to span horizontally between the intersecting walls. The walls would bend in two directions, so the wall forces on the beam would likely be quite small. However, the amount of reinforcement for the horizontally spanning beam appears to be less than the minimum requirement for beams.

I would be concerned that the amount of reinforcement in the walls, particularly in the horizontal direction, is insufficient for a water containing structure.

 

[alphaxy]

Hokie66,

I attached a spot detail of plan view.

how did you arrived at a conclusion that horizontal bars is insufficient for a water containing structure? I thought the wall could be analyzed as cantilever slab.

can you elaborate more?

 

[hokie66]

The horizontal reinforcing you have shown is only about .11% of the wall area. As a rule of thumb, most codes use around .6% for water retaining structures in order to restrict crack width.

I would suggest that your design is unbalanced in the sense that the concrete thicknesses are unnecessarily large, but the reinforcement is insufficient.

The walls can likely span the 5.6 metres horizontally without the capping beams. If the capping beams are required for other reasons, I would prefer to see them cast monolithically with the walls.

These walls can in no way be considered as cantilever walls. The walls are supported on one horizontal (bottom) side and two vertical sides, even without the capping beams.

 

[JAE]

I'd agree with all hokie66 said. If the water doesn't get as high as the bottom of the cap beam, then the construction joint would be acceptable.

The capping beams might be both a top-of-wall stiffening mechanism and also a means of getting columns (that are wider than the wall below) to connect to the wall.

 

[alphaxy]

hokie66,

Great! That is a very helpful one.

In getting the required horizontal reinforcement, (using 10mm hor bars with an area of 79mm^2) what area of the wall did you use to arrive at 0.11%? did you multiply thickness with 300mm tributary spacing?

How should the beam be analyzed? I rotated the section (610 as the width and 830 as the depth of the beam) and apply simple beam solution..Checked bending about its weak axis, shear in strong and weak axis. (Is there something that I am missing?)

For walls, what should I use to check its adequacy? Is the procedure from ACI 350 is preferable? do you have any reference, manual or guide I should look into?

I have staad model of the entire structure. However, what I just did is get the beam forces and wall forces. I don't know where to start in checking these beams and walls.

The structure is already built. I was task to do design investigation.

 

[alphaxy]

Jae,

That is a great idea. I thought of that too. I was thinking if the beam could be a part of the wall already as it might be a thickening part of wall to take into account the local stress of column on top.

As I observed the water on site, it is possible that the water may reach the top of beam. sloshing effect could also be an issue.

I was thinking of where should I start...

should I use bending formulas and obtain allowable moment and reinforcement to check its adequacy of a doubly reinforced beam "or" column approach, which will resist bending in 2 direction with axial load, from earthquake, wind forces etc.

Should I use one-way or two way slab solution for walls and have it restrained on its sides as per ACI or it is more correct to use PCA method approach (for multicell tanks design)

 

[alphaxy]

Cont..

Or I may analyze the wall as a retaining wall.

Lots of solutions, playing on my head. just don't know where to start and what is the correct approach.

 

[msquared48]

This is either a stormwater detention tank or system in the basement of a building, or a water ballast containment system, probably former. However, it could also be a means to store water for fire suppression.

In either case, from the layout provided, the walls are more likely than not cantilevered from the footing, whatever shape it is. A detail of the wall connection to the footing would help showing the shape and extent ot the footing.

That being said, I would start by looking at the wall as supported on four sides, like a plate.

I assume that all the cells fill up equally with water as the stormwater progresses?

I would also look to the details for lateral holes between the cells to equalize the water level of the system. The interior walls may not have to be waterproof, only the exterior ones.

Mike McCann
MMC Engineering

 

[hokie66]

alphaxy,

As this is a completed structure, before volunteering further advice, I would request that you advise the reasons for your investigation.

 

[alphaxy]

Guys,

Thanks for your comments and suggestions, it really helps a lot.

Btw, Is it possible to have a bending moment in major axis of beam? I pressume that It should not carry Mz because of the existing wall below, but it does. And torsion is very significant. Do I need to release the out of plane moment of walls?

 

[JAE]

alphaxy, if the beam cannot deflect(bend) about the major axis of the beam, then there will be no moment. Without deflection there is no moment.

 

[alphaxy]

JAE, yes I understand your point and I absolutely agree with that but how is that happening, I have a wall in my model, have it meshed and the beams were also cut into length of each mesh, how come it absorbs moment in strong axis? what I cannot understand is my model in staad do not take into account the presence of walls.

One thing also, is the beam carries a very significant amount of torsion, I released the moment in walls in the direction which will contribute to torsion..with the detail I provided to you, do you think it is correct to make such releases knowing that it has construction joint in beam-wall connection?

It has 100kn/m value for moment which is very significant..I hope you could give me advice.

 

[InDepth]

What are your model support conditions? is it pinned at column supports or continously supported under the walls?

 

[hokie66]

alphaxy,
We need to be engineers, not slaves to computer programs and incorrectly constructed models.

 

[alphaxy]

InDepth,

The structure is above the ground. The columns is on top of the basin. I assigned pin condition in walls of the concrete basin. It is continuously supported in every mesh of the walls (bottom only). I assigned spring supports in walls for the soil resistance. I used foundation support and input subgrade modulus, but I observed that it has a displacement of 30mm compared if I will use fixed but condition of 2mm. So I used fixed but and release MX and MZ. The moment will now be taken by the walls and beams on top as I also released the columns on top of the beam. but in beams, I expect the minor bending will carry a greater value of moment than in major bending due to the presence of walls.

please comment.

hokie66,
Sir thank you. I really appreciate it. I know we are structural engineers NOT input engineers, I was observing the behavior of my structure, its just I cannot trace what the program does. The beams on top of walls should be fixed, and shall not carry moment in major bending..but it really does in my model =( I assigned element releases in walls so it will not transfer torsion in beams, but in my model, it really has a significant value for torsion. Also, I released the columns on top of beams, but the significant value torsion is still existing, which it should not be, as it is connected to the walls and is laterally supported. There are beams transverse (no walls below) to the beams on top of walls, I made it fix, should it be pinned? The seismic weight covers 3.5 meters below top of wall because the soil at the right side is just 1.9m above the bottom of basin but at the left side the soil is flushed on top of the beam.

 

[alphaxy]

Sirs,
With the detail I provided, is the wall should be assumed pinned to beam?