Best approach for modelling a Spanish "beam and block" floor?

[zeeblake]

In some countries, the "beam and block" floor is very common, like in Spain for example (the English edition of the EHE-08 Spanish Code calls it "One-way slabs comprising secondary beams and hollow-core slabs", and the typical cross-section can be seen in this figure). The "secondary beams" are usually precast prestressed beams, and can be either of inverted T shape (like in the previous figure), or of full I shape.

The common way for modelling this with Spanish structural software packages is with bar members that take their properties (gross inertia, cracked inertia, ultimate moment, ultimate shear, etc...) from the floor catalog published by precast floor brands. After getting the diagrams for the secondary beams, the software can choose the first type of secondary beam that passes all ULS and SLS checks.

In the case of secondary beams with inverted T shape, most of the times a reinforcement for negative moments is added at the top, and the software usually obtains such reinforcement as well.

Most residential buildings in Spain are done in this way.

How would you model this with Robot?

Thanks!! 

[Artur.Kosakowski]

Hi @zeeblake

I'm afraid a general FEM software cannot be used to design such floor directly and this applies to Robot as well 

IMHO the best approach would be to use Robot (with these floors modeled as the panels with equivalent thickness assigned) to determine the overall building behavior as well as forces acting in walls/columns and foundations and then design the floors (reinforcement) in a dedicated package separately.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[zeeblake]

Actually, the structural software packages that do this in Spain are FEM-based. The procedure is very similar to the way Robot works with steel profiles: you get the section/material properties from the steel profile catalog. Then you apply FEM and get the diagrams and deformations. Then, you can check if the chosen profiles pass ULS/SLS checks, and if not, the first profile in the series that passes all checks is chosen.

It's the same, but for floors with pre-cast concrete elements that also come in a catalog. Additionally, extra reinforcement for negative bending moments is usually added by the software.

[Artur.Kosakowski]

Hi @zeeblake

Yes, it can be FEM based (for internal forces calculations) but my assumption is that the design  module for this type of slabs is an additional 'external' calculation engine which just uses these results as the design IMHO needs to be run in steps:

1. Steel beams with loads from self weight of the concrete and ceramic part of the floor with no composite action of the later

2. Composite action of the whole slab for reinforcement calculations against all dead and live loads

This would be similar to the composite beam design extension in Robot.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[zeeblake]

You are talking about the detailed modelling of the composite action within the slab. That task usually belongs to the brand designing the precast floor solution (in their catalogs they already provide you with the stiffness and ultimate strengths considering the composite action of precast+in_situ, and these catalogs have legal status, you are legally entitled to consider the provided values as true).

So, what structural software does in Spain is to simply read catalogs of precast slabs (provided by the precast maker in some standard or proprietary format) and use them directly.

The same applies to hollow core slabs: we use catalogs that have all the composite values already precomputed, and the software uses those catalogs. In Robot, if I'm reading correctly how hollow core slabs are modelled, it's quite low-level, like if you were designing the slab. This would make it difficult to model prestressed hollow core slabs, because Robot doesn't have complete support for prestressing. On the other hand, if you take ultimate values from a precomputed catalog, it's easy to support prestressing even if your software doesn't really model prestressed concrete.

NOTE: In my previous messages, I said that the software adds reinforcement for negative moments. However, it doesn't "add" it in the same way as when calculating the reinforcement in beams, but just goes to the catalog of the precast floor, and goes to the table for negative flexure... there it gets the ultimate moment and shear that can be resisted, depending on the negative reinforcement that is added.

ADDITIONAL NOTE: In the following links you can see examples for the "catalogs" that I'm saying:

Precast floor with prestressed joists

Hollow core slab

As you can see, when these tables are provided in a computer-readable format, the software has everything it needs for choosing the slab, by modelling it as a bar for each secondary beam, and going to the table with the obtained moment and shear diagram values. No need for modelling the composite behaviour (unless when you work for the precast factory... which obviously needs to model it, in order to get these tables).