On Compressive Membranes

System behaviour in fire

Something that isn’t often appreciated is that when a building is designed to achieve a 90 minute fire rating it doesn’t mean the building is designed to remain standing for 90 minutes. This might seem odd, however if we were to pose the question what size of fire is to be resisted for 90 minutes, it is immediately obvious that there is complexity involved. 

In reality a 90 minute fire rating means that the fire resisting components in the building have been tested in a furnace against a standardised fire lasting for 90 minutes. This allows the relative behaviour of different materials to be tested, however it tells us absolutely nothing about how long a real building will survive subject to a real fire. 

This is in part because fire tests treat standardised components individually, however real components are not a standard size and they act as part of a system not as individual elements. If we are being really picky we might also argue that real fires are different to standardised furnace tests.

It follows that if the structural behaviour of a system subjected to fire can be understood then this can be harnesses en lieu of the rather crude prescriptive approach, which is normally applied.

The floors of many modern buildings are constructed by casting a thin slab of concrete on a corrugated metal deck. The concrete is reinforced using a light steel mesh and the metal deck spans between down-stand steel beams. Often metal studs are welded to the top of the steel beams and are embedded in the concrete. This is known as composite construction, because the steel and concrete act together.

To protect the steel from fire the prescriptive approach is to coat it with a fire resting coating, normally intumescent paint. Intumescent paint swells when it gets hot forming an insulating layer, which prevents the steel from over-heating. Without this insulation layer steel looses significant strength and stiffness at approximately 500 degrees.

If, however, the structural system is taken into account many of the beams may not require intumescent paint. This is beneficial because intumescent paint is expensive. 

In the example shown there are primary beams joining the columns together to form a series of identical bays with two secondary beams in each bay. If we were to suppose that the secondaries are unprotected then we can begin to think about the load-path in a hypothetical fire.

As the fire becomes increasingly hot the secondaries will also become hot and will start to loose strength and stiffness. Eventually they will have little residual capacity. When this happens the floor will begin to sag and instead of supporting the concrete floor the beams will hang from it due to the embedded studs. This process is likely to be accelerated by thermal expansion which causes the beams to buckle as they push against their supports.

Conversely the primary beams, located on the column lines, are protected and will remain unaffected by the ensuing fire. They continue to form a rigid frame around each structural bay. As the floor sags it begins to tug on the primary frame simultaneously pulling each side of the bay towards the middle. Much of this work is being done by the light reinforcing mesh embedded in the floor.

This effect causes a compressive ring to be set up in the concrete at the perimeter of each bay. This ring starts to resist the floor’s tugging and allows a point of equilibrium to be reached where the weight of the hanging floor is balanced by the compressive force in the concrete ring. Although the floor has displaced significantly it has not collapsed and has therefore maintained its integrity. The fact that it has displaced significantly is not materially important, as the sole aim is survival. After a major fire a building would not expect to survive completely unaffected.

This load-path means that some of the primary beams must carry additional load, which was previously supported by the unprotected secondaries. This is acceptable, because in the fire case it is permissible for the additional load to be absorbed by the their factor of safety.

It is also worth noting that the required load assumed for most buildings is in fact much greater than the load the floors will ever see. This means the actual factor of safety is normally higher than is assumed in the cold design.

This form of system behaviour is known as a compressive membrane and I have used it successfully to assess the fire resistance of buildings on several occasions. It is a more rational approach to fire safety than the rather arbitrary prescriptive approach, which has been used historically.

On Resistance & Reaction

Surprising qualities of timber in fire

The photograph below is relatively grainy, but is reasonably well known in the field of structural fire engineering. I first came across it at University, however I haven’t been able to determine its original source. I understand that it dates to 1906 and was taken in San Francisco, which could imply that it was linked to the large and devastating earthquake of that year. 

That said, as the sub-title for this post suggests this is not a post about earthquakes. Any relevance to the San Francisco earthquake would be due to the widespread fires that it caused.

That is because the image above pictures a fire damaged roof with steel beams sagging over a supporting timber, which has, by contrast, retained its structural integrity. This is rather interesting, because it defies the common sense view that steel is a superior material to timber in a fire. Self-evidently something else is going on.

For a number of years the timber industry has been promoting timber’s credentials as a green material, however recently I have noticed that it has become rather more active in promoting timber as a good fire resisting material. It has almost become a cliche that timber is better in fire than steel. The photograph above would certainly appear to support this claim. 

I suspect that this new found advocacy for timber’s fire resisting qualities is a response to a number of recent fires; tragically some with significant loss of life.

Yet somehow the facts don’t quite seem to fit do they. Steel buildings don’t burn down and you don’t throw steel onto your campfire to make it burn. It is perhaps a good time to recycle the analogy I deployed in my post titled ‘On Howe Trusses Work’, for this is another case of there being several layers of understanding i.e. a deeper magic.

We should probably start by trying to define what we mean by fire resistance. This is not as easy as you might think. There are normally three attributes that constitute fire resistance. The first is load capacity. This is the ability of a structure to retain its strength and stability in a fire. The second is integrity. This is the ability of a structure to contain a fire and prevent the passage of hot gases through splits, gaps and fissures. The final attribute is insulation. This is the ability of a structure to prevent the transfer of heat from the face of structure exposed to fire to the unexposed face.

The photograph above demonstrates that in respect of load-bearing capacity an adequately sized timber will retain its integrity longer than steel. It is also reasonably clear that timber is a good insulator while steel is a good conductor. Placing your hand on the back of a steel plate placed in front of a fire would hurt more than a sheet of timber, at least to begin with.

Integrity is perhaps harder to judge. Timber will eventually burn through, however steel is more likely to open a gap by distorting. I shall declare this category a draw, as the inference one draws depends largely on the boundaries that are set.

Based on this rather crude assessment we reach the surprising conclusion that timber is indeed more resistant to fire than steel by a score of 2.5 to 0.5. The trouble is that this isn’t the whole story, because ‘fire resistance’ is not the same as ‘reaction to fire’.

Reaction to fire means how easily a material can be ignited and how much it contributes to the growth of a fire. On this measure timber unquestionably fairs worse than steel. Steel is of course classified as a non-combustible material.

It follows that when we talk about fire resistance in engineering we are not actually talking about the common meaning.

Using engineering definitions it may be concluded that during the ignition and growth phase of a fire ‘reaction to fire’ is the more significant material property, however once a fire is fully developed ‘resistance to fire’ becomes more important. Thus, whether steel or timber is considered better depends on timing.

That said, if we must decide then I would choose reaction to fire as more important, because resistance becomes moot if a fire doesn’t start in the first place. That is not to say that resistance isn’t important, because a steel building doesn’t stop someone from leaving a pile of combustible material in the lobby, which could catch fire. 

Another consideration would be the continued existence of historic buildings. It would not be a good idea to simply demolish those which are made of timber, because of the perceived fire risk. We need tools to understand how they will behave when subjected to fire so that mitigation can be established.

There are of course many other examples that could be considered, however these are sufficient to demonstrate that structural fire engineering is a complex business that requires a proper examination of all the relevant factors in a given circumstance.

Incidentally the reason why timber has good resistance to fire is related to its insulating properties. When timber burns it starts to char at a predictable rate. As the charring layer forms it insulates the timber beneath helping to prevent ignition at depth. For this reason steel fixings in timber can be problematic, because they conduct heat into the core of a section.

Since the rate of charring is known an engineer can calculate, for a given duration, a sacrificial thickness of timber that will be charred, while leaving a residual section of timber capable of supporting the required load.

Rates of charring do vary between timber species; the predominant factor being density. Broadly speaking the denser a wood the slower its rate of combustibility and its associated charring rate. 

This knowledge, while sounding modern, is not new. Jarrah, a hardwood [1] was commonly used for railway sleepers in tunnels and on bridges, because of its slow charring response to hot tinders falling from steam engines.

It is worth noting, however, that the relationship between density and fire resistance is not universal. Some woods have chemical contents that make them more likely to burn, however even in such cases density does tend to be the more dominant factor.

[1] it is a common misconception that hardwoods, as the name would suggest, are by definition harder and denser than softwoods. This is a useful general rule, but is not strictly true. Balsa, which is used for model aeroplanes, is a hard wood, while the rather strong pitch pine is a softwood.