Structural Engineering for Dummies

[Obviousman]

That's what I'm looking for. I talking with some people about the collapse of the towers. The claim about no steel framed building ever collapsing due to fire is raised again. We've mentioned some of the examples, but a hard-core truther (with no engineering background in any form) pooh-poohs them because they are only 2 or 3 storey buildings.

I raised this question:

The number of floors raises an interesting issue. Is a taller steel-framed building MORE or LESS susceptible to collapse if involved in a fire?

For instance:

Let's assume same building construction, same fire, only the number of floors in each building is different.

Building A: 3 storey building (ground floor, floor 1, floor 2), fire on floor 1.

Building B: 15 storey building (ground floor, floor 1, floor 2, ..... floor 15), fire on floor 1.

Which building is more likely to collapse? Building B has a greater load on it, so is it the more likely in the above example?

My layman thinking says B has a greater load above where any weakening is taking place, and therefore would be more susceptible to collapse.

I have no engineering (or physics) background, so it is just a guess.

Can someone who DOES have training in this field tell me which is correct and why?

Thanks!

 

[Newtons Bit]

It's not an easy question to answer.

A 3 story building typically has no fire protection on the beams and columns. While a 15 story building will have 1 or 2 hour ratings on all beam and columns (I don't know the exact number offhand). Fire engineering isn't used on short buildings because it isn't necessary: the people inside have plenty of time to get out before the structure is compromised and the fire fighters will have time to get in and put it out. This obviously means that 15 story buildings are less likely to collapse from a fire.

If you were to construct to identical buildings (except for column sizes) with the same fire protection, one 3 stories tall and another 15 stories, collapse would be equally likely, or perhaps slightly less likely in the 15 story building as larger column sizes are naturally more resistant to the effects of fire.

If we're talking about a global collapse (the whole building coming down), a failure in the first story of the 15 story building would be much more likely to cause a global failure of the whole building than on the 3 story.

 

[Obviousman]

Danke. So my example was a poor one to choose?

 

[boloboffin]

Here's something I finally grasped hold of during the past week.

7 World Trade dealt with lateral loads completely different from the Towers. In the Towers, the perimeter columns dealt with 100% of lateral loads. However, 7 World Trade had all lateral loads fed into the core.

I'm sure that the rest of you are looking at me, thinking, way to catch up, bolo. But having understood that now, I'm wondering: could this difference be key to understanding why the buildings fell so differently? The lateral loads of the towers are channeled across the structure to the other side -- outward. But the lateral loads of 7 World Trade are channeled to the core -- inward. Could this have "weighted the die," so to speak?

 

[Mr. Skinny]

It's not an easy question to answer.

A 3 story building typically has no fire protection on the beams and columns. While a 15 story building will have 1 or 2 hour ratings on all beam and columns (I don't know the exact number offhand). Fire engineering isn't used on short buildings because it isn't necessary: the people inside have plenty of time to get out before the structure is compromised and the fire fighters will have time to get in and put it out. This obviously means that 15 story buildings are less likely to collapse from a fire.

If you were to construct to identical buildings (except for column sizes) with the same fire protection, one 3 stories tall and another 15 stories, collapse would be equally likely, or perhaps slightly less likely in the 15 story building as larger column sizes are naturally more resistant to the effects of fire.


If we're talking about a global collapse (the whole building coming down), a failure in the first story of the 15 story building would be much more likely to cause a global failure of the whole building than on the 3 story.

(bolding mine)

I assume by your use of the term "fire engineering" you are somehow discounting automatic sprinkler systems, fire pumps, fire walls & parapets, Class I roof decks, etc. , otherwise, your statement is wildly inaccurate.

 

[Newtons Bit]

(bolding mine)

I assume by your use of the term "fire engineering" you are somehow discounting automatic sprinkler systems, fire pumps, fire walls & parapets, Class I roof decks, etc. , otherwise, your statement is wildly inaccurate.

I mean firewalls and sheathing/spray-on protection fire beams and columns. Most short-rise buildings lack these in serious forms. It's typically just the typical gyp-board that the architect uses to fur out the entire building (which does have some rating).

If they're adjacent to other buildings they'll have fire protection in the form of rated roofs, parapets, decks, walls, etc etc. I don't live in that dense of an area so I typically don't see these things in my experience.

 

[Architect]

I mean firewalls and sheathing/spray-on protection fire beams and columns. Most short-rise buildings lack these in serious forms. It's typically just the typical gyp-board that the architect uses to fur out the entire building (which does have some rating).

I suspect that what you mean is that in the USA it isn't typically used. You will find that UK and, I would anticipate, the other European systems encated under the Eurocodes would in fact demand various structural protective measures - primarily to ensure the safety of fire fighting personnel, given anticipated evacuation times of less that 5 minutes - together with compartmenalisation and various smoke control measures.

To put this in perspective, even a block of flats (that's probably a condo or something for you yanks) would require a minimum of 1 hour protection for integrity between the various units.

 

[DC]

in a 1 floor high building near me, the roof is supported with steel trusses, the trusses have no fireprotection on them, only the paint is "non flamable" so in case of fire, the paint is not spreading the fire. but no other fireprotection is protecting the steel.

i would say Newtons Bit is right oc.

 

[Architect]

How do you know it's not an intumescent paint?


Feel free to apologise once you find out what intumescent fire protection is and how it works.

 

[DC]

How do you know it's not an intumescent paint?


Feel free to apologise once you find out what intumescent fire protection is and how it works.

i saw them painting it, and i saw it when it was dry, and afaik a intumescent paint will get thicker. and i didnt see that.
but in fact, i dont know it exactly. you should know it better oc and you worked around here. its a Migros, a shop, a small one, and only one storey high.

 

[Architect]

There are a couple of things here.

First of all, an intumescent paint will just appear to be a fairthly stiff (i.e. thick) paint on application and is otherwise generally indistinguishable to the untrained eye. I assume that you didn't run up with a paint guage to check at the time.

Secondly, it might just be a class 0 paint finish, i.e. to control the spread of flame rather than actively protect the substrate. WHy might that be, however?

Well, you've put your finger on it. It's a small building, and it's only single storey. Under these circumstances the UK regs would only look for an hour, in anticipation that the firemen would stand outside and play water over it rather than actively become involved in an internal action. Why? Well, it's extremely unlikely that anyone will be inside or above the fire, and collapse will be localised.

The Eurocodes would not, IIRC, look kindly upon larger or taller structures - for example the 2-3 stories mentioned elsewhere in this thread - without more robust protection.

 

[DC]

There are a couple of things here.

First of all, an intumescent paint will just appear to be a fairthly stiff (i.e. thick) paint on application and is otherwise generally indistinguishable to the untrained eye. I assume that you didn't run up with a paint guage to check at the time.

Secondly, it might just be a class 0 paint finish, i.e. to control the spread of flame rather than actively protect the substrate. WHy might that be, however?

Well, you've put your finger on it. It's a small building, and it's only single storey. Under these circumstances the UK regs would only look for an hour, in anticipation that the firemen would stand outside and play water over it rather than actively become involved in an internal action. Why? Well, it's extremely unlikely that anyone will be inside or above the fire, and collapse will be localised.

The Eurocodes would not, IIRC, look kindly upon larger or taller structures - for example the 2-3 stories mentioned elsewhere in this thread - without more robust protection.

my eye is indeed untrained when it comes to paint

and i already said in my firt post, that it is a 1 storey building.
and i also said that i think it is a "non fire spreeding paint".
but i cannot for sure exclude a intumescent paint, but i think i would have noticed it.

 

[Mr. Skinny]

I mean firewalls and sheathing/spray-on protection fire beams and columns. Most short-rise buildings lack these in serious forms. It's typically just the typical gyp-board that the architect uses to fur out the entire building (which does have some rating).

If they're adjacent to other buildings they'll have fire protection in the form of rated roofs, parapets, decks, walls, etc etc. I don't live in that dense of an area so I typically don't see these things in my experience.

Thanks for clearing that up. I used to work as a fire protection engineer for an insurance mutual. We used to have a saying that the only building occupancy that didn't require sprinkler protection was storage of "pig iron under water".

I primarily inspected industrial occupancies where story height was typically low (usually a single story), but where hazards varied greatly. Most of my efforts were directed toward reducing property damage and business interruption rather than focusing on life safety issues.

 

[Newtons Bit]

I suspect that what you mean is that in the USA it isn't typically used. You will find that UK and, I would anticipate, the other European systems encated under the Eurocodes would in fact demand various structural protective measures - primarily to ensure the safety of fire fighting personnel, given anticipated evacuation times of less that 5 minutes - together with compartmenalisation and various smoke control measures.

To put this in perspective, even a block of flats (that's probably a condo or something for you yanks) would require a minimum of 1 hour protection for integrity between the various units.

Yes, townhomes and the like have to have 1-hour fire ratings between the different units. This can be a pain in the butt for us because we're limited by what material we can use for the shear wall. For whatever reason, my architects don't like to use OSB in a fire-wall (probably because there's no rating for it). And we structural engineers hate using gyp for a shear wall with earthquake forces on it.

For a bit of a constrast, I've had two buildings finish design and start construction recently.

One of them is an ambulance facility that has three separate buildings abutting each other, one office area (dispatch etc) and two garage type buildings. Each building is completely independent and has to have a fire-wall separating it from the other buildings, as well as fire rated parapets/roofs (I forget how the architects dealt with it). It's a one-story buildings.

The other is a two-story office building. The only fire-rating is on the floor deck which is a 1-hour rating (not the roof deck), we get that just from the thickness of concrete used, and the mechanical chases and elevator shaft walls. There's no other fire protection on the structural supports.

And then I'm also working on an 8 story office building that has to have all the columns and beam framing encased in a rated gyp-board, As well as a two-hour rating on all the floors and shafts.

Oh, and I have another building that I'm working on at the moment that is a two-story school. Alot of the structure is actually exposed. There is zero protection on those elements.

What I was getting at is that short buildings don't require the beams and columns to be directly protected while taller buildings do. I don't know the cut-off by code, the architects typically deal with this. I just ask them for weights

 

[Architect]

Yes, townhomes and the like have to have 1-hour fire ratings between the different units. This can be a pain in the butt for us because we're limited by what material we can use for the shear wall. For whatever reason, my architects don't like to use OSB in a fire-wall (probably because there's no rating for it). And we structural engineers hate using gyp for a shear wall with earthquake forces on it.

I should note in passing that timber framed domestic properties are traditionally fairly limited in extent in the UK and Europe, although they've become very popular in Scotland for detached dwellings, and that traditional construction (i.e. masonry) is more common where one has to deal with issues such as fire resisting party walls.

Internally, 1 hour fire protection is usually obtained by the simple expedient of 2 layers of broken bond (i.e. staggered jointing) 12.5mm plasterboard. Thereafter in a joister floor we would go up to specific fire protection boards, usually one of the Cape Fireline or equivalent.

One of them is an ambulance facility that has three separate buildings abutting each other, one office area (dispatch etc) and two garage type buildings. Each building is completely independent and has to have a fire-wall separating it from the other buildings, as well as fire rated parapets/roofs (I forget how the architects dealt with it). It's a one-story buildings.

Liimited risk of entrapment in a fire, ready means of escape, hence the garages are relatively straightforward. The compartmentation requirement is quite sensible. If it had been one larger structure I would have anticipated more onerous requirements to control the spread of fire and smoke.

The other is a two-story office building. The only fire-rating is on the floor deck which is a 1-hour rating (not the roof deck), we get that just from the thickness of concrete used, and the mechanical chases and elevator shaft walls. There's no other fire protection on the structural supports.

Mechanical chase? In the UK a chase is a groove cut in a wall to accommodate wiring or small diameter piping (in Scotland and Northern Ireland also called a raggle). Clearly you don't mean that.

And no, a building like that in the Uk and Europe would require significant fire protection. Probably 2 hours, IIRC, depending upon occupancy levels and use class.

And then I'm also working on an 8 story office building that has to have all the columns and beam framing encased in a rated gyp-board, As well as a two-hour rating on all the floors and shafts.

We'd be looking at a higher standard in order to ensure access for fire fighting and also safeguard adjacent properties.

Oh, and I have another building that I'm working on at the moment that is a two-story school. Alot of the structure is actually exposed. There is zero protection on those elements.

We would be on a minimum of 2 hour protection plus class 0 surfaces, together (of course) with smoke control and a full autodetection and warning system.

What I was getting at is that short buildings don't require the beams and columns to be directly protected while taller buildings do. I don't know the cut-off by code, the architects typically deal with this. I just ask them for weights

Typical ruddy engineer.......

 

[Minadin]

By Mechanical Chase I'm sure Newton means the various shafts we cut in the buildings for the HVAC duct-work to pass through from floor to floor. Those are required to be rated by code.

We also are required to fire-rate certain things depending on the use and class, and certainly between tenants in a multi-use building, and also especially between floors, but there are certainly cases in shorter buildings were none might be required. Also there are many cases where a sprinkler system is allowed to substitute for a 1-hour rating.

 

[Architect]

By Mechanical Chase I'm sure Newton means the various shafts we cut in the buildings for the HVAC duct-work to pass through from floor to floor. Those are required to be rated by code.

We'd just call that a services penetration (oh-er, missus) and yes, we would require to fire proof it to the equivalent standard to the wall/floor - usually through the use of intumescent collars and fire dampers, although it is possible to instead use a fire rated duct/box-in.

(snip).....Also there are many cases where a sprinkler system is allowed to substitute for a 1-hour rating.

Ah, no, we don't allow anything lower than 1hr rating. A sprinkler is treated in the Scottish and (IIRC) the English regulations as being an "extra" item which can only be used if there's a problem elsewhere requiring remedial mitigative measures (for example an existing building, where not all finishes might be class 0).

 

[Newtons Bit]

Typical ruddy engineer.......

I'm just doing my best to further exacerbate the relationship of engineer and architect everywhere I go

 

[uk_dave]

i saw them painting it, and i saw it when it was dry, and afaik a intumescent paint will get thicker. and i didnt see that.

Intumescent paint only gets thicker when subjected to fire, otherwise it looks fairly ordinary.

You will also be oblivious to intumescent seals applied to the frames of fire doors you may encounter. You probably won't notice those seals and hopefully won't get to see those seals swell up and seal around the door when there is a fire.

ETA: An example of a company 'truthers' seem to believe doesn't need to exist... http://www.nullifire.com/

 

[DC]

Intumescent paint only gets thicker when subjected to fire, otherwise it looks fairly ordinary.

You will also be oblivious to intumescent seals applied to the frames of fire doors you may encounter. .....

oh i didnt know that. i thought it gets thicker when its drying.
well then i dont know what paint they used. and i dont gonna try it
thx