Structural assessment after a fire

Structural assessment after a fire

After a fire the building is usually still standing, and that becomes the assumption everyone works from. It is the wrong place to start.

The question is not whether the structure survived the fire. It is what capacity it has left, and that is a materials question that the appearance of the building does not answer.

Steel

Structural steel loses strength as it heats and regains most of it on cooling, which is why a steel frame that went through a moderate fire is often reusable.

The exception is distortion. A member that has buckled, twisted or sagged has yielded, and that deformation is permanent. It also means the connections at each end have been loaded in ways they were not designed for.

Cold-formed steel, purlins and light gauge framing, behaves differently and loses capacity at lower temperatures.

Bolts and welds need their own assessment. A connection that has been through a fire can look sound and have lost preload or developed cracking.

Concrete

Concrete is more complicated because the damage is often internal.

Above certain temperatures the cement paste changes chemically and does not recover. The concrete may look grey and intact from outside while having lost a substantial share of its strength through the affected depth.

Spalling is the visible sign. Water in the concrete turns to steam and pushes the surface off, which exposes reinforcement directly to the fire and removes the cover protecting it.

Colour gives a rough indication of the temperature reached, and testing gives the answer. Core sampling for strength, and assessment of the depth to which the material has been affected.

Reinforcement

Reinforcing steel inside concrete that stayed cool is generally fine. Reinforcement that was exposed by spalling, or that sat in a region that got hot, may have lost strength permanently.

Cold-worked reinforcement is more vulnerable than hot-rolled, and prestressing steel is the most vulnerable of all. A post-tensioned slab that was exposed to fire needs specialist assessment, because the tendons can lose a large part of their capacity at temperatures well below what the concrete around them tolerates.

Timber

Timber chars from the outside and the char layer insulates what is beneath it, so a heavy timber member can retain useful capacity after a fire.

The assessment is about residual section. Measure past the char to sound timber and check whether what remains is adequate. Connections are usually the weak point, because metal fasteners conduct heat into the timber around them.

Masonry

Masonry generally performs well, and the issues are cracking from thermal movement and from the water used to fight the fire, and loss of mortar strength.

Walls that were restrained at the top and heated will have moved, and the movement may not have reversed on cooling.

What the assessment produces

A map of the fire, where it was hottest and how long it burned there, from the physical evidence and from the fire service record.

A materials assessment of each affected element, with testing where the visual evidence is not conclusive.

A statement of residual capacity, element by element, and what has to be replaced, strengthened or can remain.

That last part is what insurers and certifiers need, and it is what turns a damaged building into a defined scope of works.

Do not let anyone occupy or demolish first

Two things go wrong immediately after a fire. People go back in before the structure has been cleared, and demolition starts before the assessment records what was there.

Both are avoidable. Make it safe, keep it out of use, and get it assessed before decisions are made.

We carry out this work under structural engineering, and the assessment usually finds more of the building is reusable than the owner expected, and a different part of it is worse than they thought.