Diaphragm Action And How Floors Brace A Building

A building's lateral stability is usually described in terms of its shear walls or its core. Those are only half the system.

Wind does not arrive at the core. It arrives on the facade, at every level, and something has to collect it and carry it horizontally to the elements that resist it. That something is the floor, acting as a deep horizontal beam.

What the floor is doing

Think of each floor as a beam lying flat.

The wind load along the facade is the distributed load on it. The shear walls and cores are its supports. The floor spans horizontally between them, in its own plane, developing bending and shear exactly as a vertical beam does.

Because it is very deep relative to its span, a concrete floor is usually enormously stiff in that direction and the check is straightforward. Which is why it is so often not checked at all, and why the cases where it fails are the ones nobody looked at.

The two jobs

Collect and transfer the lateral load to the bracing elements. This is the main one.

Tie the vertical elements together so they act as a system and so each is restrained at every floor level. A column's effective length depends on this, which is why it feeds into [columns, slenderness and buckling](/blog/columns-slenderness-and-buckling), and a wall's out-of-plane stability depends on it too.

A floor that cannot do the second job leaves slender walls and columns unbraced over multiple storeys.

Where a diaphragm actually struggles

Large openings. A big void for an atrium, a stair or a lift group reduces the depth of the horizontal beam, and if the opening cuts most of the way across the plan the floor can no longer span. The remaining strips either side have to carry the whole force, and they have to be reinforced to do it.

A long thin plan. A floor plate with a high length-to-width ratio is a long span beam in plan, and its in-plane deflection stops being negligible. That deflection means the far end of the building moves more than the braced end, which loads the elements unevenly.

Re-entrant corners, on an L, T or U shaped plan, where the two wings want to move differently and the corner is where the force concentrates. The most common place for diaphragm cracking.

Bracing elements all at one end, which makes the floor a cantilever in plan and introduces torsion in the building as a whole.

Precast floors without an effective topping or tie, where the individual planks can slide relative to one another. A precast floor acts as a diaphragm only because of its connections, and this is where the integrity provisions in [disproportionate collapse and tying requirements](/blog/disproportionate-collapse-and-tying-requirements) and the details in [precast connections and site tolerances](/blog/precast-connections-and-site-tolerances) do structural work.

Timber and steel deck floors, which are far less stiff in plane than a concrete slab and need a designed bracing layer, whether that is a structural screed, plywood sheathing or a steel bracing system.

The collectors nobody draws

The force has to get from the floor into the wall, and that transfer happens over the length where they meet.

Which means reinforcement running into the wall, anchored, sized for the shear being transferred. On a core with a short connection to a large floor plate, that transfer is concentrated and it is a real design item.

Drag bars, or collector reinforcement, carry the force along the floor to the point where it can enter the wall. They are ordinary bars doing a specific job, and they get left out whenever the diaphragm is assumed rather than designed.

What changes in an existing building

Diaphragm capacity is removed by the alterations people make most often.

Cutting a new floor opening for a lift or a stair, which is exactly what [adding a lift to an existing building](/blog/adding-a-lift-to-an-existing-building) involves. Creating a double-height void. Removing a wall that was a bracing element, as in [removing a load bearing wall](/blog/removing-a-load-bearing-wall-what-it-takes). Demolishing part of a floor plate during staged works, which is a temporary condition covered by [demolition sequencing and structural stability](/blog/demolition-sequencing-and-structural-stability).

Each of those can be acceptable and each needs the in-plane check, because the consequence is not a local sag, it is the building's lateral system losing its connection to the load. That is a whole-structure question, and it is the reason a floor penetration is never a site decision, as set out in [core drilling and penetrations in existing slabs](/blog/core-drilling-and-penetrations-in-existing-slabs).

During construction

The diaphragm does not exist until the floor is complete and connected.

Which means the part-built structure has a different lateral system from the finished one, and the bracing has to be provided temporarily. This is the condition behind the brace requirements in [temporary bracing of precast panels](/blog/temporary-bracing-of-precast-panels) and it is the reason a structure part-way up is more vulnerable to wind than the completed building.

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