Stresses within a glass and concrete slab 1

[cve60069]

Dear Engineers

I am wanting to check the design of a reinforced concrete-slab that has glass-block voids and a band of insulation (please see attached). The slab is constructed in two parts; with the glass-blocks laid out, the bottom rib, which contains the reinforcement, is poured first and the dowels fitted. The insulation is pushed over the dowels when the concrete is sufficiently set and the top layer of concrete is poured.

The slabs will be expected to span 2.5-m (11-blocks x 11-blocks)

I have assumed that the the dowels and concrete-blocks will allow the slab to act as a whole and that the mode of failure would be through the glass-block where it meets the concrete (X-X in the diagram).

The insulation will be in the 'tension' area of the section.

The section would be evaluated by taking moments around the N.A. The glass will only be expected to resist shear. Dowels are to be used to hold the two skins of concrete together.

Would I be right in my analysis of the mode of failure, please?

Regards

 

[dcarr82775]

I am no sure what you think the failure mode is. At ultimate strength the insulation may be below the plastic neutral axis, but at service loads it may not be, and at pre-cracked section properties some of the insulation is above the neutral axis.

Where you have the insulation, you have two individual 45mm thick slabs/beams, not a 110mm thick section. The dowels across the insulation need to transfer horizontal shear to make the two pieces composite, and they will simply bend across the gap. You can back calculate a composite (110mm) section strength based on what horizontal shear you can transfer through bending of the dowels. It will not be very much.

 

[rapt]

I do not know what you think the dowels are doing. They are useless.

I do not think the top and bottom concrete can be assumed to work together as a slab the way it is detailed.

If you got rid of the insulation, it could work as a ribbed slab!

 

[cve60069]

The dowels are to hold the top and bottom sections of concrete together, nothing more. I got the idea from a previous thread I submitted on this learned forum. Wires were suggested to hold the skins together but we decided on pre-cut dowels as less fiddly.

You seem to be ignoring the glass-blocks. The blocks have a known structural strength and will provide the stability required, including in the transverse direction. The blocks will have to resist the strain of deformation, not the insulation. The strength of the insulation is irrelevant; only its location is important.

The neutral-axis will be calculated for the cracked section with only the compressive-concrete and tension-steel being considered in the equations but, as the glass-blocks are holding the whole slab together, the blocks need to be checked for failure but not factored into the functions. I think the blocks would be the primary failure mode; a brittle fracture below the neutral-axis. The block would not fail where encased in concrete. The walls of the glass-blocks may be 20-mm thick but I doubt if the steel would fail first.

As for not using insulation and casting the rib in concrete - that's what I do every day for a living. I make pavement-lights. This insulation is being requested more and more in my work and I am trying to assess the strength of the slab. I blame global-warming for this extra work.

Regards

 

[hokie66]

Agree with rapt. The glass blocks are just voids structurally.

 

[concretemasonry]

The glass blocks are not compatible structurally with the cast in place concrete from a structural standpoint and even thermally if you are in an area with major temperature swings between the construction dates and the actual conditions. - They are just a strong "void" in the slab and the insulation and amount of steel between the two slabs to make them act together.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[rapt]

The dowels will not develop any force as they have basically no development into the concrete, so they are essentially useless. They will pull out if they are subjected to any real force as they would have to be if they are to transfer shear between the top and bottom concrete elements as is required to "hold them together".

The glass blocks are voids as Hokie said!

 

[dcarr82775]

The get the composite action you want using the glass blocks you need a real provable bond between the glass and the concrete. I wouldn't think that bond is very good. Also, it looks like there are (2) pieces to the glass block. If that is the case you still need to transfer horizontal shear to make the two blocks act compositely.

You have NO composite section.