25 Story Apr Bldg Coming Down in Seattle

[msquared48]

Check this out:

http://www.thesunbreak.com/2010/04/11/who-killed-belltowns-mcguire-the-cast-of-characters

The article points to a structural firm I worked for in the mid 80's. Further points to a water ingtrusion problem as the cause of the problems.

This is an expensive mistake for a lot of people, and will not be the last found I fear from the list of projects mentioned.

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

 

[BAretired]

I think that unbonded post-tensioned systems should be outlawed. It is virtually impossible to prevent corrosion of the strands inside plastic sheaths.

BA

 

[msquared48]

Additionally, I think that stucco should be outlawed in wet (humidity higher than 20% average) climates. I have seen too many problems with it over the years.

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

 

[structuresguy]

Wow. That is pretty scary scenario. I feel for the structural engineer on this one for sure. I have personally designed about a dozen buildings using post tensioned slabs. I would hate to be dragged into such a scenario by an architectural waterproofing failure.

I have to say though, that I rather agree with BAretired. I would rather not design PT slabs ever again, as there are just way too many problems associated with them.

 

[dik]

I've used bonded and unbonded PT systems lots of times and nary a problem... going back 40 years...

Dik

 

[ron9876]

Need to use fully encapsulated cable and be specific about the grout to use to seal stressing ends.

Must not have been due to water leaks since PT garages are fairly common without problems.

 

[drawoh]

A few years ago, all sorts of condominiums in Vancouver were built with leaky roofs. Is the rain in nearby Seattle is comparable to that of Vancouver?

JHG

 

[graybeach]

I often get involved with maintenance of PT and prestressed structures, including several PT girder bridges and a parking garage. Our structures are 30+ years old, and they definitely do have problems with corroded and blown out tendons and strands. It's very difficult to repair these problems, and failure of tendons and strands seriously reduces capacity.

I would like to see provisions for localized member replacements built in to design documents. This way wholesale replacement of structures might be avoided or delayed. Seeing what I have seen, I really wonder what the future holds for PT box girder bridges that have tendons in the deck slab.

 

[msquared48]

Drawoh: Yep...Seattle's only a couple of hours south if Vancouver.

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

 

[kelowna]

Graybeach,

PT on bridges typically has a minimum of 3 level protection. they are grouted inside galvanized steel ducts embedded in the concrete deck.

 

[abusementpark]

Where do you guys figure the corrosion is occuring? At the tendon ends?

 

[msquared48]

The tendons were not treated with anti-corrosion inhibitors as is the norm. So, in addition to the ends, anywhere along the tendon. In that lies the problem. Fingers are pointing everywhere here.

The building was valued at 60 mil, but the repairs 80 mil.

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

 

[abusementpark]

Do you know if the EOR called the corrosion inhibitor in their specs or not?

Do you know if they used an encapsulated system at the ends?

 

[msquared48]

I have not heard that they did spec it yet. The implication in the local report was that no one had covered the issue, which does not bode well for the structural. It may have been an oversight, assuming standard practice, which it is, but... tell that to the lawyer.

Don't know if the system was encapsulated either...

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

 

[abusementpark]

It is standard practice not the require a corrosion inhibitor on the tendons?

 

[BAretired]

abusementpark,

The strands are normally covered with grease which is presumably a corrosion inhibitor, but in the past, the grease has emulsified over time allowing corrosion to occur.

Water enters the plastic sheath and flows to the low point of the strand which is usually about midspan and that is where corrosion is likely to be found.

Sometimes the guys on the job are pretty careless about the way they drag the plastic sheaths over reinforcement. It is not uncommon to find a hole in the plastic sheath where it rubbed against something. All it takes is one little hole to let the chlorine ions in.

BA

 

[msquared48]

Yep...

And I thought this was an interesting quote too:

"The building's owners — a joint venture between a Seattle Carpenters Union local and a union pension fund."

The present and future pensions of a lot of locals are going to be affected here too. Not good for business...

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

 

[abusementpark]

Is a regular reinforced concrete slab just as susceptible to corrosion from water intrusion? Or is it the sheathing in the PT slab that traps the water and causes corrosion?

 

[BAretired]

Conventional reinforcement is covered on the ends with 2" of concrete, so there is no open duct for the water to enter.

Also, the smallest reinforcing bar is much larger than one wire of a seven wire strand. The exposed area per unit volume of material is much less for a bar, making it less likely to corrode than PT strand.

Finally, PT strand is stressed to very high levels throughout its life in the structure. This makes it subject to stress corrosion cracking which is not an issue with conventional reinforcement.

BA

 

[msquared48]

Apparently the water intrusion is not the only problem here. In structural report 1 as listed in the link below, it discusses the compromising of the exterior special concrete moment frames due to omission and misplacement of certain critical tie bars. This is a BIG ouch!

http://www.themcguire.com/

Scroll below the pictures to the 6 engineering reports listed and read - very interesting.

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