Web holes and service openings

The structural side

The web carries most of the shear capacity of the section and plays a part in bending too. A hole in it:

• reduces the shear capacity — most of all near the supports, where the shear force is large,
• can cause local instability around the hole,
• also reduces resistance to web crippling if it is near a bearing.

Hence the regulated solution: holes of a given size, in a given position, at defined distances relative to the length of the section and to the supports.

A factory service hole cut to the program follows that logic — which is another reason it is better than drilling on site.

What must never be done

• Cutting a hole afterwards in the web without the engineer’s approval.
• Enlarging an existing hole.
• Passing through near a support.
• Cutting through a flange — the quickest route to losing the capacity.

The building-physics side

Here comes the more important argument. Machine makers like to advertise that holes can be cut in the profile for cables, and many people take them up on it. In practice this is a bad idea for two reasons.

Fire

A cable passing through a fire-separating element would need a verified seal. Fittings exist on paper; in practice they are almost never installed because of the cost — so the solution exists on paper and not in the wall.

Vapour control

This is the more serious one. In light gauge steel construction vapour control is done solely by the membrane. Puncture that envelope anywhere and moisture will collect there — and a hole drilled for every light switch is exactly such a puncture.

The solution to use

Fire-rated board and the vapour control membrane go on the inside face of the structure, and the mechanical and electrical installation runs inside that, in a second plane formed with battens. Service cavity.

The vapour control layer then stays intact, the sound insulation improves, and a later strip-out only has to open the inner board — not the structure, the insulation and the membrane.

The point

A web hole is manageable structurally and a risk in building-physics terms. The good solution is not to punch holes well but not to need them.