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How to Design Capping Beam ? 4
- Thread starter MHJ35
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Normally the best design would come from a 3D model that contemplates the effects of long term settlement, if any is expected (floating piles). For the cases where long term settlement is not seen as a particulary worrying issue the same 3D model of the box with the cap beam atop it may be enough, on some elastic support (you may add push and supporting floors for a more perfect model), always with the settlements (or soil stiffnesses) attuned to what expected. Applying the column loads to this model will give you the design moments and shears.
The restraint provided by the ground against the walls if very true is of difficult evaluation and is normally (and conservatively) left out of the model.
It is not too uncommon to feign fixity conditions on that beam (so passed to the model of the superstructure above, if separate) to reduce the amount of torsional forces applied. In any case a mechanism needs to be provided to ensure proper transmission of loads to the foundation.
The restraint provided by the ground against the walls if very true is of difficult evaluation and is normally (and conservatively) left out of the model.
It is not too uncommon to feign fixity conditions on that beam (so passed to the model of the superstructure above, if separate) to reduce the amount of torsional forces applied. In any case a mechanism needs to be provided to ensure proper transmission of loads to the foundation.
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MHJ35, engineers have been designing beams for many, many years without using FEM or 3D analyses. You need to provide some information on what you are trying to do. Why do you need a cap beam? What are you trying to accomplish?
I have seen in closely supported micropiles (not just secant piles) designs going from the weak cap beams to the quite rigid. Continuous-alike and quite rigid support may account for the weak cap beam cases, whereas the rigid designs should be based in some feared unreliability of the support that it is wanted the rigid cap beam counteract.
So I would say that very rigid cap beams come mainly from lack of confidence of a regular behaviour of the foundation. Exception made of cases where the piles end in layers of vast different rigidities, and surmised one has reasonable investigation of the soil, what uses to be common for this kind of works, most engineers won't go for the top rigid cap beam, but neither are them providing very weak ones, just in case.
So I would say that very rigid cap beams come mainly from lack of confidence of a regular behaviour of the foundation. Exception made of cases where the piles end in layers of vast different rigidities, and surmised one has reasonable investigation of the soil, what uses to be common for this kind of works, most engineers won't go for the top rigid cap beam, but neither are them providing very weak ones, just in case.
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PEinc,
Secant piles wall is required to support excavation. This wall shall be 'water cut-off wall' also. This wall will be located within airport premises so the owner and consultant requires capping beam connecting the secant piles wall (male & female). I need to submit simple approach like if the horizontal component of the active pressure equals (x) for example then i will use a reasonable ecom=nomical safe percentage of it on the cap beam of length say 1.0m and furnish the structural design (section, steel bars, concrete strength ...etc). Does this simple approach seems reasonable and acceptable by the consultant? Does this way of thinking had been used earlier according to your knowledge?
Ishvaaag,
I would like to thank you very much for your interest but as explained above, do you have simple "used before" design approach for temporary shoring works?
Secant piles wall is required to support excavation. This wall shall be 'water cut-off wall' also. This wall will be located within airport premises so the owner and consultant requires capping beam connecting the secant piles wall (male & female). I need to submit simple approach like if the horizontal component of the active pressure equals (x) for example then i will use a reasonable ecom=nomical safe percentage of it on the cap beam of length say 1.0m and furnish the structural design (section, steel bars, concrete strength ...etc). Does this simple approach seems reasonable and acceptable by the consultant? Does this way of thinking had been used earlier according to your knowledge?
Ishvaaag,
I would like to thank you very much for your interest but as explained above, do you have simple "used before" design approach for temporary shoring works?
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Unless the capping beam is tied back, it is just going along for the ride. The width will be controlled by the width of the secant piles plus an overhang both sides, the height can be whatever looks right. Reinforce it to prevent unsightly cracking.
MHJ35, since the times I have had to design cap beams there have been in the time-frame of PC-computed assisted engineering I have always dealt with the issue in the terms in the first answer exposed. Of course if no slab counteracts the earth push atop the wall in the height of the cap beam (and even if exists) provision needs be made for the push there, some of which can come from adjacent footings and be locally very strong. Sometimes for this one uses Boussinesq's formulations for the push, other simplified closed form statements of the push.
When I have seen design of others as a general rule the effects of long term settlement (maybe a device of the cases I saw) is usually neglected, and inwards (towards the excavation) movement corresponding to the push is restrained by ties to the ground for temporary situations or the built slabs for the definitive. I really do not remember any quoting a guide book on the matter, they just deal with what think relevant at the moment.
When I have seen design of others as a general rule the effects of long term settlement (maybe a device of the cases I saw) is usually neglected, and inwards (towards the excavation) movement corresponding to the push is restrained by ties to the ground for temporary situations or the built slabs for the definitive. I really do not remember any quoting a guide book on the matter, they just deal with what think relevant at the moment.
I agree with hokie66. The cap beam does nothing unless some secant piles need to receive some support from the adjacent piles by way of the cap beam which could act as a wale. What would your design load be on the cap beam?
For example, a sheet pile wall is designed on a per linear foot basis. Each sheet pile supports the same load as long as the wall height, soil properties, and surchargre loads remain the same. Therefore, if you add a cap beam to the sheet piles, it will not perform any useful, structural function. As hokie66 said, the cap beam will just go along for the ride when the sheet piles (or secant piles) deflect. If someone of authority wants a cap beam, you can give them one. I'm just not sure what load you would need to design it for.
For example, a sheet pile wall is designed on a per linear foot basis. Each sheet pile supports the same load as long as the wall height, soil properties, and surchargre loads remain the same. Therefore, if you add a cap beam to the sheet piles, it will not perform any useful, structural function. As hokie66 said, the cap beam will just go along for the ride when the sheet piles (or secant piles) deflect. If someone of authority wants a cap beam, you can give them one. I'm just not sure what load you would need to design it for.
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What's the size of the excavation. If the distance between the intersect walls are short you could use the perpendicular walls as support points and design the capping beam as a simply supported beam spanning horizonatally with a uniform (or nearly uniform) load imposed by the top reaction of the secant piles. You can also detail the intersecting cap beams at the corners to take moment.
Hi all.
We are doing a secant walled pile rectangular basement. We are not able to install anchors and the wall needs to be designed as a cantilever wall. Horizontal deflection of the wall governs the design of this specific wall and I'm trying to provide restraint to the top of the wall using a stiff capping beam and am wondering what the best way is to calculate the equivalent spring stiffness that the capping beam will provide to the top of the wall.
My current approach is modeling a rectangular capping beam in isolation, applying a unit load to all four the sides of the basement and extracting the deflections this induces and relating back by k=1/deflection*width to give me a kN/m stiffness that the geotech boys can apply to their frew model.
Sound right?
Thanks
We are doing a secant walled pile rectangular basement. We are not able to install anchors and the wall needs to be designed as a cantilever wall. Horizontal deflection of the wall governs the design of this specific wall and I'm trying to provide restraint to the top of the wall using a stiff capping beam and am wondering what the best way is to calculate the equivalent spring stiffness that the capping beam will provide to the top of the wall.
My current approach is modeling a rectangular capping beam in isolation, applying a unit load to all four the sides of the basement and extracting the deflections this induces and relating back by k=1/deflection*width to give me a kN/m stiffness that the geotech boys can apply to their frew model.
Sound right?
Thanks
If the secant pile wall is truly cantilevered, the cap beam is probably doing nothing to support the wall. The cap beam will move with the secant piles as they deflect. If the cap beam is a four sided bracing frame, it will provide support for the secant piles which will therefore not be cantilevered piles. The piles can't be both braced and cantilevered at the same time.
PEinc, the basement is recatuglar in plan so you have a four sided capping beam frame which will provide a degree of restraint depending on the length. Closer to the corners provides higher restraint and at the midspan you would get the least. So the analogy is to model a cantilevered wall with a horizontal spring at the top simulating the capping beam. Have you done something like this before?
Not like you describe. Your rectangular secant pile wall is really just a braced, four-sided cofferdam. I would design the wall as a braced wall (not cantilevered) and calculate the required bracing load per unit length of wall. Then, using the calculated load, design the cap beam which is really a bracing wale or frame. Make the wale/cap beam/frame as stiff as you need to minimize its horizontal deflection.
Gmanskip,
The spring coefficient at top of piles will vary along the wall, minimum at midspan of capping beam and very rigid at the corners. To evaluate springs run a single span beam model of capping beam loaded with uniform load w=1 k/ft. Spring coefficient will be K = (di/w)S, where di - deflection at the pile, S - spacing between piles. Vary capping beam stiffness and beam support conditions to envelope the results. I attached a graphical illustration to this idea.
Hope it helps!
Yakpol
The spring coefficient at top of piles will vary along the wall, minimum at midspan of capping beam and very rigid at the corners. To evaluate springs run a single span beam model of capping beam loaded with uniform load w=1 k/ft. Spring coefficient will be K = (di/w)S, where di - deflection at the pile, S - spacing between piles. Vary capping beam stiffness and beam support conditions to envelope the results. I attached a graphical illustration to this idea.
Hope it helps!
Yakpol
Gmanskip,
firstly, it is more polite to start a new thread even if it is closely related to someone elses like this. But since we have started then we may as well continue.
It looks to me like you are overanalysing this. Just model it as a rigid support in order to get a slightly conservative reaction and the design the capping beam for the u.d.l.
If we were to analyse every item in a project in detail without simplifying it I would still be designing my first house (from 13 years ago).
firstly, it is more polite to start a new thread even if it is closely related to someone elses like this. But since we have started then we may as well continue.
It looks to me like you are overanalysing this. Just model it as a rigid support in order to get a slightly conservative reaction and the design the capping beam for the u.d.l.
If we were to analyse every item in a project in detail without simplifying it I would still be designing my first house (from 13 years ago).
csd72,
Simplification will be justified only if you have a small in plan excavation. With longer sides (say over 60-80ft) the forces on capping beam are very substantial, and flexibility of it is important too, you may exceed allowable displacements at midspan.
Simplification will be justified only if you have a small in plan excavation. With longer sides (say over 60-80ft) the forces on capping beam are very substantial, and flexibility of it is important too, you may exceed allowable displacements at midspan.
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