Load sharing floor and beams problem
Load sharing floor and beams problem
(OP)
Hello
I am having to design a thin (60mm) reinforced-concrete slab that will be supported on a grillage of one-way spanning beams (Picture) and I need to check the whole for a concentrated 100kN load in the centre of the slab. The concrete and steel are free of each other.
I am assuming that the slab will be able to spread the load across the joists and the maximum deflection will be beneath the point-load.
I need to check the stress and deflection in the concrete and steel and am not sure how to proceed. Any help in analysing a load sharing structure would be appreciated.
Regards
I am having to design a thin (60mm) reinforced-concrete slab that will be supported on a grillage of one-way spanning beams (Picture) and I need to check the whole for a concentrated 100kN load in the centre of the slab. The concrete and steel are free of each other.
I am assuming that the slab will be able to spread the load across the joists and the maximum deflection will be beneath the point-load.
I need to check the stress and deflection in the concrete and steel and am not sure how to proceed. Any help in analysing a load sharing structure would be appreciated.
Regards






RE: Load sharing floor and beams problem
RE: Load sharing floor and beams problem
We are not paid to assume, we are paid to justify these things to the best engineering knowledge at the time.
Sounds like you need a mentor on this one as this is too broad a question to be adequately covered in a forum.
RE: Load sharing floor and beams problem
RE: Load sharing floor and beams problem
Mike McCann
MMC Engineering
RE: Load sharing floor and beams problem
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Load sharing floor and beams problem
I am not checking the slab. The slab will be as thick as required; I am checking the behavior of the joists. I am not convinced that the load will be shared between two joists but over all the joists, not equally. Tests on similar structures show the loads exceed 100kN but the math do not confirm this. I am trying to understand the influence the two structures have on each other.
Regards
RE: Load sharing floor and beams problem
RE: Load sharing floor and beams problem
Respect how to analyze it well, you will need previously the max range of the loading slabs themselves, the dead weight may be significant enough to influence the design of your steel part.
RE: Load sharing floor and beams problem
If I treat the top slab as a plate with all sides simply supported (see pdf). The slab being supported of a series of simply supported beams. The plate and beams are free to rotate. The beams are restrained horizontally and are close together.
Would I be safe to assume that the ribs would act as a plate with an EI equal to the EI of a rib multiplied by the number of ribs.
I would allow the plates to deflect 1mm (say) and calculate the P from plate deflection formula where
Plate deflection = rib deflection = 1
Once I know P, I can then calculate the deflection and stresses.
Am I right or very wrong?
RE: Load sharing floor and beams problem
Is this a real structural issue or a research problem? If you can provide the actual design parameters, perhaps we can suggest a solution.
RE: Load sharing floor and beams problem
However, when continuity is a problem, or the slab stiffness relative to the support below is weak, one can't assume much of the load goes to any other part than to what below, such for reactions and shears when the wheels reach the supporting parts in bridges, or like when in your case you have the load over one rib where there is a joint in the slab and the slab is as weak as to not be able to significantly affect the forcibly conjoint deflection there, what most would suspect is the case when the load is 10 ton and the (concrete) slab thickness 60 mm.
In any case, the proper set of FEM analyses with the proper interfacing parts' restraints and constraints should reveal any aspect you are interested in.
RE: Load sharing floor and beams problem
The design is a real problem. It is a smoke-outlet from a basement. Normally designed spanning one-way but are cast supported on four sides. Its the way the outlets are built. See newageglass.co.uk - Merchants Square or Cafe Royal for example.
The fire-brigade have to smash through the slab in 7-blows. The outlet ranges from from 80mm to 130mm thick and 1200mm wide. The slabs have to support 100kN (75kN in London) - not direct but over an area of 300mm square. Rib centres are 150mm or 165mm. We are going to build some and have them tested (hopefully) at Kingston University. I am trying to predict the behaviour of the slab and design an optimum rib. The ribs are normally concrete but I am also designing an outlet in steel and concrete but I need the slab to spread the load for the ribs to be "up to it". The lights deflect very little, 2 - 3mm.
For now, I am wishing to write down the formula to allow me to calculate the stresses in the structure, regardless of the materials - or - create an FEM of the structure. I would prefer the hand-method as I do not own any FEM.
Oh! and the slab is waffle.
RE: Load sharing floor and beams problem
RE: Load sharing floor and beams problem
This is a very good forum. Thank you all.