Question about pre-dimensioning

[MOHAMMED]

hi everyone ..

I have a Question:

when we design a building , how do we get its dead load as we do not have the dimensions from the beginning ? i recall one of my instructors said that we generate a preliminary design then we modify it .. can someone elaborate on this ??

I need a clear answer ..
thnx

[ishvaaag]

Normally any building design starts by some architectural definition. A proficient architect is able to specify both tentative dimensions and structural systems convenient to structural safety and good use, even if he himself is not to produce later the structural design.

Then any structural designer has something with what to start; some specific kind of foundation, structural system, structural devices will be pre-defined; and some non-structural items will be stated as the components to cause dead loads; some uses will be specified for the parts of the building.

Fron such data and resourcing to code load data, say ASCE 7, you define the loads for your building. Of course such loadings are derived from previous experience on the particular items referred in the articles and are thought to lead to economically bearable, consistently safe and socially accepted standards of building construction.

[Dell_Brett]

Dear Mohammed,

It's usual starts a design with some practical dimensions. For example: -for concrete beans, we starts with the equivalent height of 8% to 10% of span.
After pre-dimensioning, we check if deformations, moments, shear forces, etc, are compatible with our preliminary pre-dimensioning.
Now, we starts a try-and-error changes until the structure is OK!
Naturally, experienced structural engineers can do this with only few interactions...

If you need more explanations, please returns with clear points that you don't understand.

Regards

Dell Brett

[seb3k]

As dell said.
you can find tables with pretty much standard dimensions of elements.
like for beams... span/10
for solid concrete slab... span/35...add some standardized additional dead loads for flooring, lets say 0.75kn/m2,
add live load as per code...such as 3kn/m2 for residential spaces, and you have a good starting point