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Let's say you build a square reinforced concrete deck at height of, say, 3 meters, with the deck supported by its own steel reinforcement and by four reinforced concrete piers (call them, A, B,C, and D) below each corner of the square.

If you then build four infill walls, A-B, B-C, C-D, D-A, are those walls under compressive load? From one side, the weight of the concrete slab would say yes, but on the other side the fact that deck was supported prior to the walls, maybe not?

Is there a difference, in the context of the loads transmitted to a wall below, between a vertical stack as follows:

wall - reinforced concrete ring beam - slab

and

wall - reinforced concrete deck with reinforcement designed to support the deck by itself and the piers alone.

Here's an example, without the ring of beams:

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  • $\begingroup$ Unless there is an air gap (or weatherproof airgap i.e. rubber expansion joint/caulking) I would consider the walls to be load bearing. They don't have to be the strongest member to be a load bearing feature. $\endgroup$
    – Dopeybob435
    Commented Jun 8, 2016 at 16:48

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It depends on the order of operations for the construction of the structure.

Let's start assuming the structure is as simple as the image you've given us: a single story with a slab supported by columns.

If the columns and slab are built, their formwork is removed and only then the wall is built, the wall shouldn't suffer any stresses. This is because the removal of the slab's formwork allowed it to deform itself into a stable configuration. When your wall comes up to the slab, it will simply close the gap, but won't have to support the slab since, well, the slab is already supporting itself. This will only be true in the short term, however. Over time, creep will try to further deflect the slab, and the wall will end up resisting that movement, generating a compressive force. This can be reduced by increasing the time between removal of the formwork and the raising of the wall, thereby increasing the amount of creep which occurs without harming the wall (this effect would be minimal, though).

If the columns and slabs are built and the wall is raised before their formwork is removed, then the wall will suffer compression. In this case, when the formwork is removed, the wall will (try to) impede the deflection of the slab and will therefore be compressed.

If the slab is actually supported by a ring of beams, the same effects apply as described above. That being said, the beams will dramatically increase the stiffness of the slabs, and will therefore reduce the deflection of the structure above the wall. This will reduce the compression suffered by the wall (either immediately or due to creep), but not eliminate it.

If the structure actually has multiple floors, then the entire structure should be built before any walls are fully raised. The walls should also be raised starting from the top floor, and then proceeding downwards. This way, the load due to the wall of an upper floor will already have deformed the structure before a wall is raised. This way we can guarantee that a wall will not participate in supporting the walls above it. If the walls are made of brick, these can be raised along with the structure, so long as they leave a gap between their top course (row of bricks) and the bottom of the slab. Once the entire structure is raised, that gap can be filled (starting from the top floor).

As @AndyT well mentioned in a comment below, regardless of when the wall is raised, it will participate to some extent in resisting loads due to live/accidental loads.

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  • $\begingroup$ So presumably: (a) if you build a ring beam (without a slab on top) directly on top of a wall, then the wall suffers compression from the weight of the beam, but this is minimal because the beam is very stiff (b) if you build a slab directly top of a wall, then the wall suffers more compression than in case of (a), because the slab is much heavier than the beam, and because the slab is going to be less stiff than the beam. $\endgroup$
    – thelawnet
    Commented Jun 8, 2016 at 15:00
  • $\begingroup$ (a) If you build a beam directly on top of a wall, the stiffness of the beam won't matter because, while the beam is still curing, the wall will already be supporting its weight. Therefore, when the beam dries, it won't have to resist any forces because the wall is already resisting them. (b) If a slab is built directly on a wall, however, it will also have formwork supporting it away from the wall. Once that formwork is removed, the entire slab will deform and will be resisted by the wall, generating compression. $\endgroup$
    – Wasabi
    Commented Jun 8, 2016 at 15:16
  • $\begingroup$ 1. What will compression do (as in bad things) to a wall? Or does this depend on the wall construction details (e.g., the mortar has higher compressive strength than the masonry unit, or vice versa)? 2. Some wall types may be classed as 'load-bearing'. E.g., in Thailand 20cm thick AAC blocks are deemed 'load-bearing' Does this mean something specific in terms of the load on top (which could very substantially in weight)? $\endgroup$
    – thelawnet
    Commented Jun 8, 2016 at 15:29
  • $\begingroup$ and 3. if you built the same slab with four piers, but the wall in this case was of a construction standard normally deemed 'load-bearing' (even though the slab is already fully supported), then does that affect the order of construction? Or is it still desirable to avoid compressing the wall, even though it is of 'load-bearing' standard? $\endgroup$
    – thelawnet
    Commented Jun 8, 2016 at 15:43
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    $\begingroup$ Important addition: if the infill wall is built afterwards, it won't carry any permanent/dead load from the slab; but it will still carry live/imposed/variable load from any people or items placed on the slab at a later date. $\endgroup$
    – AndyT
    Commented Jun 13, 2016 at 14:27
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Since you are building infill walls you don't expect the wall to resist compression. You must calculate the structure without considering the walls as resisting elements, only as weigh to the beam below it. Usually in this cases the mortar between the bricks is expected to be flexible enough that if the structure deforms the wall is able to deform with it and not resist any significant tensions.

Also for this purpose when you build an infill wall with ceramic bricks you always leave an empty space between the wall and the structure above it (~2cm) that is only filled when the whole structure is built (upper levels) and the initial deformations of the reinforced concrete have occurred. Even so, when you do fill this space, you will want to fill it with some sort of flexible filling (sometimes mortar is enough) so that the future deformations of the structure are absorbed by the wall.

So usually you don't expect the wall to be able to support any tensions, but there will be some due to the deformation of the reinforced concrete structure. For that reason brick walls are considere flexible and must be able to accommodate this displacements without resisting.

All I've said is considering you don't want the wall to be structure components (you did say they were infill walls).

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