The Effect of Fire on Property and the Environment [24:40]

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En provenance du cours de University of Maryland, College Park

The Effect of Fires on People, Property and the Environment

103 notes

University of Maryland, College Park

The Effect of Fires on People, Property and the Environment

103 notes

Fires impact people, property and the environment in all countries around the world. In some cases, the resulting losses are extraordinary, causing hundreds of deaths, widespread damage to property and contents and significant impacts on the environment. More often, fires may cause a single casualty or affect a single home, though the effects are still highly significant to those affected and collectively are substantial. This course will provide an overview of the challenges posed by fire as well as the fire safety solutions that are available to meet those challenges.

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Effects of Fire on Property and the Environment

The Effect of Fire on Property and the Environment [24:40]24:40

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James Milke, PhD
Professor and Chair
Department of Fire Protection and Engineering

Well Hello.

Wanna provide another segment in our course on The Effect of Fires on People,

Property and the Environment.

And today the particular topic at hand is to

look at the effect of fire on property and on the environment.

So the outline for this particular week will involve

looking at an impact of fire exposure on structures to see how a structure,

and in particular the load bearing components, will respond to fire.

That is, respond to a heated environment posed by a fire..

And then secondly, we'll wanna take a look at the impact of fire, and smoke, and

extinguishing agents on the contents of the building and also on the environment.

So our first piece is to look at the effect of fire exposure on structures.

There is some information in the literature that's

available through the Fire Protection Engineering Magazine published by SFPE.

There is a website that provides the articles that is open to the public

and would encourage you to take a look at these articles for

a little more information.

They're relatively short articles that can be read fairly easily, not usually real

intense in the way of the scientific principles that are involved and

such that it provides some very nice background and gives some insights into

the engineering aspects in our field here, or in this particular topic of our field.

So the first question we wanna pose before we get into the details of looking at

why structures potentially fail, is to look at so why is this important?

And there are a few reasons why it's important.

Certainly looking at just the overall issue of property protection

is important and if a structure collapses because of fire, that usually

then has associated with it a very large property loss as well, dollar value wise.

But in addition that there are issues of

wanting to have the structure stay up in order for

the building occupants to be able to evacuate prior to the building collapse.

And as emergency responders are dispatched to

the building if there is a fire incident.

Having the building stay up and not collapse while they burn,

to responders are in the building is a very important issue of course.

There are different ways that we protect structures,

that's part of what we'll talk about here.

We'll mention with the array of buildings here.

So there are a few of these buildings,

where the life safety aspect becomes an issue, and that usually is the case

in highrise buildings that take a fair amount of time to evacuate, so

as we look at the new tower here, get myself oriented, sorry,

get the new tower in the mideast, that is a couple of thousand feet tall.

That's gonna take a long time for

people to evacuate from the top level on to the ground level.

The First Interstate Bank building in Los Angeles and

the fire that occurred in 1988.

Another high-rise fire.

And there weren't many people in the building at the time of the fire, but

wanting to preserve the structural integrity of that building to allow

building occupants to get out.

And for that particular incident, for emergency responders to get in and

control the fire was an important issue.

There are other buildings, like the cable TV headquarters for China.

Being here, being a rather innovative structure.

The Olympic bubble For

the swimming competition at the Beijing Olympics.

We have our US capitol, where there's more than life safety,

now if that building burns down there is this historic building that

has cultural significance, would be lost.

So there's those issues.

I do have the basketball arena here as well on campus.

Where there's a life safety issue.

Where there's 17.000 people watching games.

And it's going to take awhile to evacuate that building.

Having the roof collapse while the people are trying to get out would not be,

at all, a desirable outcome obviously.

And again, there's the issue that if there is a serious fire in this building,

would the emergency responders be able to get in, and not to have to worry about

the structural collapse, would be a very important issue.

So, one of the values we want to talk about is,

is the same level of fire assistance provided in all structures.

And there's a rather easy answer to that, and the answer is no.

It is said that one doesn't have the same level of fire resistance in

all structures.

There are some structures that have no formal fire resistance rating associated

with them and there are others such as the high-rise buildings that we looked at that

may have several hours of fire resistance that would be designed into them.

In terms of the US codes and

standards, the basis for the US codes and standards and basis for

the fire resistance requirements in those standards really is expressed here.

And, you can see this comes

out of a 1942 report from the National Bureau of Standards, at that time.

It's now called the National Institute of Standards and Technology, and there was

a report, the BMS reports were building material series as they were called.

For the BMS 92, one of the classic studies that were done in the early part

of the 20th century says here the subcommittee that was

tasked to consider fire resistance requirements believes that the idea of

designing some buildings for the full fire severity corresponding to the occupancy

is a logical advance in fire protection engineering.

So what that means is that the buildings should be,

at least in principle following this they should be able to withstand a full burnout

of all the materials in the space, all the materials in the space would be consumed.

And the building should still be standing.

A few years prior to this BMS report, Simon Ingberg

put together a hypothesis of how long a fire will last,

what was its severity measured in hours, according to Ingberg's hypothesis.

And this was dependent upon the fuel load.

And in how big of a building area was involved.

Fuel load is one of the characteristics that helps to find

the occupancy of the building that is, the use of the building.

And is that building a warehouse, or is it a restaurant,

is it a library, an office, all have different amounts of fuel in them.

And basically the more fuel,

the longer one would expect a fire to stick around or

to burn, and that's also dependent on how big of an area

is there to burn before you achieve the burnout.

So we have these different levels of fire resistance then

are required according to the building codes and

standards in North America and I think applied to many countries.

Similarly.

So as we look to then achieve a particular level of fire resistance you say, so

how does this work, how do you do this?

Well we want to first look at,

so what happens to structural elements when they heat up.

And I'll say the one common item that we can make, we can say for

all structural materials, whether it's steel or concrete or masonry or

timber elements or whatever the structure is composed of,

all of those structural materials lose strength with temperature.

There isn't one of them that is able to

heat up to whatever unlimited temperature and maintain its structural integrity.

So the question becomes then,

how quickly does the strength decay or decrease with time?

I'm sorry, with temperature,

that may relate to time in terms of fire continuing to burn.

So how much of it decreases there and

at what temperature do significant changes start to occur?

For steel, I show a graph here of the yield strength,

or the strength of steel in terms of its load carrying ability, and

also the modulus of elasticity, where the modulus of elasticity is

a parameter that's used in engineering to address how

easy is it to deform or deflect to bend a particular structural member.

Those are two separate issues, two separate strength, and

how easy it is to bend a particular material or to get it to deform.

And you can see for steel there is this gradual decline, so it starts up,

at its ambient temperature strength, there is gradual decline of temperature, and

in terms of strength characteristics, if we look at the solid line,

we have about half of the strength, as compared to ambient temperature,

up around, well let me draw it on here.

So we're about half the strength, and

we're somewhere in the neighborhood of the mid 500s C,

where steel has lost about half of its strength, as an example.

So if we knew the safety factor from the structural engineers is to

how heavily loaded the structure was going to be,

how heavy the steel structure would be in this case, we could estimate a failure of

temperature, at least in a very simple sort of approach.

And there are much more complicated approaches we'll allude to here briefly.

But that gives you an idea, maybe a very simplistic idea,

of what sorta temperature limits there might be for a structural member.

On the photo up at the top right, this is a photo from World Trade Center Building 5

associated with the events of 9/11, where the building sustained a very significant

burnout on several floors of the building, the fire was allowed to burn,

firefighters didn't go in and attempt to control the fire on that particular day.

This building wasn't hit by aircraft,

other than maybe a few bits of debris so this is basically a fire vent.

You can see the steel is heavily deflected, significantly deflected,

a little bit of flange buckling up in the top issue here, and

this is a very common scene for a steel frame building

that has some fire resistance in it, that had two hours of fire resistance in it,

in ways that we'll talk about in a couple minutes, but shows you what

can happen, potentially, as a result of a very significant fire exposure.

What you see here is there's no collapse, and it's relatively rare that high

rise buildings collapse as a result of fire, and that's why the events of 9/11,

or one of the reasons, why the events of 9/11 are so significant.

Where there were three buildings that collapsed that day, the twin towers and

World Trade Center 7, where fire had a piece of that,

or had a part of the reason for the collapse.

Wood and concrete also are affected by elevated temperature.

Wood will lose strength with temperature as well.

One of the significant characteristics about wood,

in addition to the strength reduction, is that it chars,

which you all appreciate already without having enrolled in this course.

The charring that you see in the photo here, the charring and the depth of

the char relates to how much heating there was, what was the duration of the heating,

what was the magnitude of the heating that gets a particular char depth.

As that wood element chars, this char layer is unable to support a load.

So we're now losing load carrying ability as a result of that exposure.

The inner part of the wood here is maintained relatively cool,

cuz wood has some insulating abilities that are enhanced in fact, by the fact,

that there's now a char layer, so that Mother Nature was kind to wood in

the sense that, as it heats up and chars, it provides a bit of a protective layer at

least, in terms of limiting how much heat transfer and therefore how much heating,

how much temperature as there is in that interior portion.

Concrete, again, has will lose strength to a temperature so

it's not fireproof, there is no fireproof material structurally.

The Pentagon here and the collapse of the Pentagon building as again a result

of the event on 9/11 indicated on this slide, where they can see

some collapse here that some of it was a result of aircraft impact, but there were

collapses as indicated here as a result of the subsequent fire and the fire exposure.

So how do we protect structural elements?

Well, there's a lot of interest in finding ways to keep them cool or

to delay the heating process similar to that char layer on the wood.

For steel elements you see the photo with the spray applied fire protection material

that's there, or insulating material sometimes called fireproofing,

which is perhaps not the best word again, because there's nothing really fireproof.

But it does delay the heating process for the steel, and presumably,

or at least philosophically, long enough to allow the fire to burn,

consume all the combustibles, and still have the steel stay up,

similar to World Trade Center 5 such as the photo I showed you.

In addition to insulating materials,

there's the possibility of providing a heat sink perhaps.

And this is a case where a heat sink would be,

such as a concrete encasement around a steel column,

such as you see here, that will provide some insulating abilities.

Of course, concrete's a pretty good insulator.

But it also has this heat sink ability.

And what a heat sink does is it is able to take in a lot of heat

without itself increasing in temperature a lot.

And that's why the label heat sink gets used.

There are other ways of protecting structures, and

some of this may be to reduce the exposure so that as we,

as you apply a strategy to put the fire out,

well then that takes away that heat exposure, I think obviously.

So there may be suppression systems that will get applied, and

these get called active protection methods, as opposed to the passive methods

that are the categories of insulating materials and heat sinks.

That's all passive because it's always there, it's sitting there.

And doesn't have to be activated by a detection device for example.

So the sprinklers shown here would have to be activated of course,

but that water spray would be able to cool the environment,

maybe control the fire, maybe put out the fire.

Venting systems also get applied in some warehouse sort of environments,

for example, to release some of the smoke or heated gases if you prefer

from that fire area, and that again limits the exposure that the structural elements

sees that that roof supporting member would see.

So it's another way of providing protection.

And then the whole issue in providing fire resistance or providing protection for

the structure ends up being a bit of a balance between passive methods and

active methods.

And how to provide that balance I will say is an issue of

much debate in that how much do I rely on these methods,

the so called passive methods that are, that don't have to be activated.

How much do I rely on these.

And there's again the arguments that have gone on in the literature for many,

many decades, and I'm sure will go on for many, many more,

as to what the right balance is in terms of effectiveness, in terms of reliability,

both these systems, the active systems, and these systems have to be maintained.

So you don't get any free pass cuz it's always there.

There are issues where they're maintenance problems maybe with the integrity of

that insulating material.

But there are lots of questions that come into the design practice

in terms of can I do less of the passive methods if I do some of these.

In going back to the concept from the committee from the BMS 92 report,

it said we should be able to withstand a full burnout.

Well, if you can prevent the full burnout from occurring,

maybe that means we don't need as much of the insulating materials.

And the building codes in the United States have accepted that principle,

or accepted that as a design option, and where the active

methods are used such as sprinklers, in particular.

Perhaps we don't need as much of the insulating material and

there might be an hour less of fire resistant material

that's needed to be provided to achieve structural protection.

There are more advanced methods by which we can examine the performance

of a structure and what's the right level of protection to go through an engineering

assessment that might allow you to go through a detailed analysis and

comparison of how much active and how much passive protection do we need that's based

based on what heat exposure is gonna be provided to the structural element and

how that structure will then respond to that particular hypothesized fire.

There are three steps to this analysis that need to get conducted.

The first piece of it is to understand the fire exposure.

So what kind of fire is going to be there?

What are the characteristics of that fire?

What sort of elevated temperatures are going to be provided?

What's the magnitude of the elevated temperatures that will be provided,

how long will that environment be present and be exposing the structure.

So there's the fire dynamics aspect of it that we have to predict the growth of

the fire, predict the behavior of the fire and get a sense for what is,

what kind of temperatures, what kind of heat fluxes are gonna be provided by

the fire, generated by the fire that the structure's gonna be exposed to.

The next step to that is then okay, so now that we know what the fire is,

we conduct a heat transfer analysis on the structural elements.

So I show here a computer simulation of a floor system that has a concrete deck,

a steel beam and that steel beam has that spray applied fireproofing material on it,

spray applied fire protection material on it would be,

again, the proper phrase to use.

And the colors all indicate temperatures that that

assembly has achieved as a result of some prescribed fire.

So this is a heat transfer analysis and

I show you one result here from this model called sapphire.

There are several models that are available,

including some that are run of the mill commercial heat transfer codes,

finite element methods that could be used to explore what temperature rise

you'd expect in the various components in the structure where the fire and

the heat conditions, the temperature conditions, the heat flux associated with

the fire end up being boundary conditions through that analysis.

After we conduct the heat transfer analysis then

that gets input into a structural analysis, where now we apply

a structural code to assess so what are the deflections that might occur?

Does the structure collapse?

Do we get buckling?

Any of those sort of structural behaviors that would get applied based on

the temperatures that are predicted here in the heat transfer analysis

where we would assess the loads, the decreases in strength, the potential

expansion of the elements as they get heated up,

etc, to be able to again assess how long does it take for

the structural to reach some failure condition,

either because of having excessive deflections, or just plain collapse.

And then to tailor the protection methods accordingly.

And this type of analysis was done for

a high rise building under construction in London that I took a photo

of several years ago where the gray indicates those

structural components which have had the spray applied material applied to them.

In this case, a coating that was used.

The red colored steel beams here

are not protected and are not intended to be protected.

They're secondary beams.

And part of that structural analysis through that finite element

calculation that I showed you on the previous slide,

would have demonstrated that if these couple of red members fail,

that the structure can still survive even though they perhaps fail and

are not carrying any load anymore, where there's load transfer to the other beams,

the columns, perhaps the floor slab carries some of the load as well.

So that advanced analysis is possible and

that is the whole item I want to make mention of here.

There are some very simple methods by which we assess how much of that spray

applied material to put on based on results from standard tests.

And then there are these advanced calculations that are possible and

have just recently become available and

used by some design engineers for selected projects.

So in summary for this segment, fire resistance levels varies as we've said,

and based on the hazard basically that is provided by that particular building,

by that particular use of the building called occupancy.

And all that's dictated on a hypothesized potential fire severity

that comes from some rules of thumb from the early part of the 20th century or

can come from elaborate computational models.

And then secondly, the other point I wanna bring up is the strength of all structural

materials decreases with temperature, and that there are protection methods that get

used to restrict the temperature rise in the structural members, and

help them withstand the insult provided by the fire.

So with that, we'll bring this to a close.

And we'll continue with another segment of this in a couple minutes.

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