Hello

The last few weeks, I deal with the slab of a METRO station. It is a rectangular reinforced concrete slab of dimensions 110mx26m.

The thickness of the slab varies from 0.6m to 1.0m. The slab will be simply connected to the perimetric diaphragm walls via dowels.

No interior vertical elements will exist. In the interior, rc beams of 2.00x1.60m (bxd) along the smallest dimension have been placed (span 26m).

Initially the point of interest is the beams.

Two calculations have been made.

In the first calculation, the structure was modeled with surface finite elements of appropriate thickness. Even the beams were modeled with fin. elem. having the corresponding thickness.The software that was used was Sofistik.

The purpose of the second calculation was to check the computer results. This was a traditional hand calculation. The beams together with various holes and openings in the slab 'panelised' the slab. Each panel was treated then as a separate slab with the appropriate support conditions. Using Czerny tables, the reactions from each panel were calculated and assigned to the corresponding supporting beam. The beam was then calculated as a simply supported beam acted upon by the said reactions. This calculation is typical for conventional buildings and has been used for many years.

The difference in the beam span moment between the two calculations was more than 50%. The fem analysis showed that in fact the slab is not panelised and behaves like a one way slab thus carrying a large portion of the load and therefore relieving the beams. The hand calculation was more unfavorable for the beams.

Which calculation is correct? Should the fem results be trusted?

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Dikaios Psarogiannis A.M.ASCE

Consulting Engineer

OMETE SA

Holargos

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