Fixing Into a Structural Floor: What to Establish First

Every one of these constructions presents a flat surface above and a continuous face below, and none of them reveals what is inside. That is the whole difficulty. A soffit gives no indication of profile, gauge or lap in a decked floor, nothing on a precast unit shows where its steel runs, and there is no visible difference between a stressed plate whose tendons are where the drawings say and one whose tendons are not.

So the first question is not where to drill. It is what this floor is, and what record of it exists.

In a joisted floor, a joist is not equally strong across its depth or along its length, so where a hole may go is decided by how the member works and not by where the fixing wants to be. The entry names the misunderstanding directly: drilling a joist is treated as a plumbing task, when where a hole may go is decided by how the member carries load.

Engineered members change the answer completely. Some have designated service openings and are compromised by a cut elsewhere, which is why the member type has to be known before anything is cut into it.

Drilling into a precast unit is assumed to be like drilling into a slab, when steel running the length of the unit changes what is permissible. Where the prestressing strand sits inside a unit is recorded by its maker and nowhere else, and there is nothing on the soffit or the surface that reveals it.

The library draws the practical conclusion: a proposal to core or to fix into such a floor rests on retrieving that record rather than on inspection, and what may be fixed into or drilled through is defined by the manufacturer and the designer rather than judged on site.

Here the sheet you would be fixing into began as formwork and ended as the tension reinforcement of the slab. Anything done to it takes something away from that role. The entry describes the soffit as doing structural and coordination work at the same time: its ribs give a hanging surface, and fixings driven into the underside meet reinforcement and, in places, the connection zone above.

What looks like a free surface to hang from is constrained by the section it belongs to, which is why where services may be fixed to a composite soffit is a question for the structural designer.

This is the one that behaves least like the reinforced slab it resembles. A penetration formed afterwards is not a hole in a slab; it is a cut tendon, and the force that tendon was carrying is released along its whole length — not within the size of the opening, and not within the bay the opening was made in.

The entry is equally clear that a heavily reinforced slab and a post-tensioned one are not the same argument. An argument that a slab is heavily reinforced says nothing about whether a tendon can be disturbed, and an argument that it is post-tensioned says nothing about whether the ordinary reinforcement can be cut.

Even in an ordinary reinforced element, a fixing is not only a mechanical question. The concrete outside the outermost bars is what stands between the reinforcement and the air, water, salts and heat on the other side of the surface, and chasing, drilling and surface damage all remove part of it.

Whether that is acceptable at a given location is described as a question for the designer rather than for the trade doing the work.

The sequence is the same for all four floors: identify the construction from the record, retrieve what the record says about what is inside it, and take the proposal to whoever is responsible for the structure. Where a tendon layout for an existing floor cannot be established, the library treats that as a finding to take to a structural engineer rather than a gap to be worked around.