Questionable Engineering Design 1

[msquared48]

From the internet headlines today:

"TODAY - May 01, 2010

(picture)

Deadly tornadoes rip through Arkansas Twisters hit central Arkansas, injuring dozens, destroying homes, and taking down power lines. Details
Arkansas tornadoes
Derby forecast improves
Your local weather"

There was a picture here I could not post of a collapsed fire station with a fire truck inside. It was a CMU building with a wood roof.

Personally, other than money, I cannot understand the logic for allowing this type of construction in a tornado prone region. Now, in the emergency condition it was designed for, the fire truck cannot do it's job.

In my opinion, it should have been constructed of CIP concrete walls with a CIP concrete roof, and blow out windows to relieve the pressure. The outcome should have been much better. To me, it's just irresponsible engineering design with a lack of forethought.

Thoughts?



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteveGregory]

Could the headlines read "Wood roof collapses and damages fire trucks" and the story says that it will take 2 weeks to repair the trucks.

Or could it read "Concrete roof collapses and destroys fire trucks". just a thought?

 

[concretemasonry]

My first engineering project was in CA where I had to design a concrete blockhouse (reinforced CMU was the standard of construction
) for instrumentation (aerospace industry). About 2 weeks later there was an accident becuase of a gas leak.

The cloud of hydrogen over the building ignited. The pressure blew the wood frame roof in and created over $1,000,000 in damage. The building was designed well above all the standards for pressure and exposure. The walls survived with a problem. Several "explosion proof/bullet proof windows" that were 4" thick where blown in and steel door was also destroyed, but he CMU walls had no problems.

It is not a question of the materials but the judgement of the professional in determining the possible load conditions for the building use. - At the time, I had only been out of school for 1 month, but I should have learned after seeing rocket test stand total destroyed on my first observation.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

Mike,
Good point. Here in Florida we have certain buildings that are designed way beyond the code requirements....hurricane shelters, emergency service centers (including SOME fire stations, but not all), emergency communications facilities and towers,etc. I've done strengthening design on two hurricane shelters taking them from code (120-130 mph) to Cat 5 hurricane (155 mph +).

In my opinion, certain emergency services and shelters should be taken beyone typical code requirements in areas where the probability is high of such an event (Hurricane prone area, tornado alley, etc.).

Ron

 

[IRstuff]

In most cases, it's a question of cost, and the willingness to gamble. That's why people still live in flood plains, tornado alleys, hurricane alleys, and earthquakte zones. Most people cannot afford to demo their existing structures and build stuff to current code, so everything is grandfathered to the code that was extant at the time of construction.

It's no different from the fact that my parent's house, which is only 84 yrs old is grandfathered to the electrical code of the time, which didn't require grounding wires, and didn't preclude aluminum wiring, and didn't preclude fusible link fuses. Likewise, the house has no sprinkler system. To retrofit would require gutting the house, and spending probably $100k for only a potential problem.

I've had similar thoughts every spring, when regions of the south and midwest experience flooding; why don't they build on stilts? Obviously, the cost, evne though the houses are relatively inexpensive there, compared to SoCal, but when compared to the standard of living, a retrofit would essentially might require a complete demo and rebuild.

TTFN

FAQ731-376

 

[woodman88]

It may have been more cost effective to build additional fire stations, spaced far enough apart, so that only some would be damaged beyond use by a tornado than to make all strong enough to survive the tornado.

Garth Dreger PE
AZ Phoenix area

 

[MikeHalloran]

Since architects and engineers are typically engaged on a per-building basis, not on a political- entity system basis, I find it implausible that such a systemwide partial survivability strategy was used, anywhere, ever.

I'm not saying that it _shouldn't_ be used.



Mike Halloran
Pembroke Pines, FL, USA

 

[IDS]

The concept of "lifeline structures" having higher design loads, and much greater resistance to collapse, than ordinary structures is an accepted part of earthquake engineering. If it isn't a part of accepted design philosophy for other exceptional loads, it should be.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[woodman88]

I believe that “system-wide partial survivability strategy” is one of the main concepts of emergency planning. As such, I would design the the fire-station to the level the client wants, while meeting the minimum requirements (the code does have higher standards for emergency buildings) of the code. It is quite possible that the above fire-station was designed per the code and the clients requirements. As such, the only question in my mind is whether the fire department properly planned for this occurrence.

Garth Dreger PE
AZ Phoenix area

 

[vandede427]

Mike, I always value your posts/advice very highly, but the phrase "tornado prone region" is just plain silly. two thirds of the lower 48 are tornado prone.

 

[msquared48]

Vande - Yea, you're right. I should have restricted it to the classic Tornado Alley region, and maybe some of the periphery regions.

Heck, we even have an occasional F1 in the Puget Sound area, but never anything higher. I guess I was thinking more of those regions regularly seeing F3 and above.

I still think though that these emergency structures should be at least partly bermed at the first story, with an earth cover over if only one story. To me, it just seems prudent.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vandede427]

I like this map.

 

[jt12]

Hurricanes and Earthquakes have much bigger coverage zones and are designed by typical building codes where Tornados are not. The worst hurricanes have wind speeds of 165 mph where tornados can have wind speeds of 200 - 300 mph. Steve Gregory and Woodman88 both make good points in that it is probably better to keep the roof light and scatter more firestations around to create more redundancy against one being taken out. I do believe nuclear structures are designed against tornados, but can't think of anything else.

 

[TXStructural]

While you could quite feasibly build a tornado-resistant fire station, the likelihood of a tornado hitting a particular fire station is infinitesimally small.

The building codes do not specify tornado loads, since doing so would be unnecessarily burdensome. Should a tornado occur, an EF1 will decommission a fire truck in the station just about as certainly as an EF5. If we mapped the probably that our homes and businesses would be hit by any tornado, the probability would be nearly zero.

In a system safety analysis, we would assess the combination of probability and severity for foreseeable events. In this case, one would project a severity of "critical" to "catastrophic" occurring with probability of "remote" to "improbable", for a risk assessment which requires acceptance at the lowest level, but little else.

If you live in tornado alley, you may or may not consider such things when buying a house. With the virtual absence of tornado-resistant housing, it is apparent that we simply accept that something might happen, but we figure it won't happen to us.

 

[plasgears]

Some managers have a bias toward wood structures. We had a snow load failure of a roof over an outdoor storage area. The responsible party got quotes on steel and wood, and he chose wood. 'The quotes were about equal, but I prefer wood,' said the man in charge. He got wood trusses over a 100'x100' area, and the last time I looked, a man was hammering nails thru the sheet metal shear plates. Piss poor.

 

[engrbon]

I'm just wondering why we are continuously building wooden structures in areas affected by extreme atmospheric conditions. From a structural point of view, it doesn't seem logical.

 

[vandede427]

because wood is cheap and easy to build. little tiny towns can't afford the engineering and/or cost of a reinf concrete structure.

 

[TXStructural]

Wood is a perfectly good engineering material. It is not the wood that is the weak part of the building, it is the poor engineering used in design, poor construction practices, and poor maintenance of the structures.

Plate connected wood trusses are an adequate construction material, but as with many things we are asked to design and build, they are typically built to the least strength necessary. This is a function of the designer, and the overall structural system design, and can easily be overcome by better selection of loading schemes, and better selection of structural features to increase structural toughness.

As for the cost of building more robust structures, public projects (and most commercial projects) typically require an engineer, so that cost should be there using any material. Reinforced concrete is not significantly more expensive than other materials, when the entire envelope is considered, but it does require the right contractors. As you point out, wood structure is easy to build to a level good enough for most users. And most "local builders" are familiar with wood, but not with proper construction methods for engineered structures.

 

[FSS]

Before I pass judgement, I would like to know if they even used hurricane clips and/or otherwise considered the entire load path. There are still many parts of the country where wind is an afterthought with only mild prescriptive requirements, especially with residential construction. Not saying anything could be done with a direct strike, but wood works pretty good when you actually tie it together.

 

[audeuce02]

I am suprised that it hasn't been questioned if the building was actually designed. It is common practice in rural areas for buildings of this nature to just be built, with no design consideration. The exception would be if the Fire Department received matching funds from the state or federal government.