Cinderblock construction

[STurner]

I have some questions regarding cinderblock construction. Are homes and buildings that are built on cinderblocks usually have a weaker foundation than those built on a slab foundation or those with a basement. It made me wonder about this recent tornado that hit Mena. There was a building that was made out of cinderblock constrction that was completely destroyed. The cinderblock construction made me wonder if it was a weak construction practice and may have prevented an EF4 rating. In addition to what I found out about how they rate tornadoes on the EF-scale is they want at least two damage indicators before justifying a higher tornado rating. For example if a very well-built brick home is swept clean off its foundation and disintegrates downwind, then this alone would probably not qualify for an EF5 rating. I believe it would be rated a high-end EF4. Now if this scenario happened and another damage indicator or some non damage indicator such as a car, tree, tractor, or something significant is extremely then that probably qualify as an EF5. I could be wrong but I went through the EF-kit and how I understood it that they want at least another damge indicator besides a home before justifying the highest rating.

 

[Steve Dedman]

they want at least another damge indicator besides a home before justifying the highest rating.

Shane,

It seems reasonable that they would need numerous indicators for any EF rating, since in seemingly equal houses/buildings, not all things are equal. In your cinder block building, for example, there can be huge variations in the strength of the mortar within the same building due to something as simple as the amount of water used in the mortar. How much cinder block area is exposed to the wind load? How much interior support is behind that exposed area? Are the blocks in a stacked bond or a running bond (one is a stronger bond than the other)? Was there reinforced concrete in the voids? Was the guy at the cinder block plant trying to remember how many beers he actually drank the night before while he's monitoring the ratio of ingredients that make the blocks? Did the masons meet code in the number of brick ties used in each course of blocks? What was the ambient temperature when the blocks were laid? All of those considerations and many more will have varying degrees of effect on the final strength of the building.

One simple design change led to the deaths of 114 people at the Kansas City Hyatt in 1981. That's why stuff is over-engineered to the point of being ridiculous to the layman - when people's lives depend on it, you have to account for multiple human foibles along the process.

Like in any complex construct, if you really stop to think about the number of variables that have to be in place to make the thing work/be safe, you'll go quite mad very quickly.

If you really want to scare yourself, ponder for a moment the number of people that all had to be sane, sober, knowledgeable, competent and conscientious in the manufacture and delivery of your car, from the time the engineer put ink to paper to the time the salesman handed you the keys, including all of the thousands of small parts manufacturers.

Are homes and buildings that are built on cinderblocks usually have a weaker foundation than those built on a slab foundation or those with a basement.

A lot of basement construction uses cinder block as the basement walls, which then rise up some distance over the ground level. For basements using concrete, the fact that it is reinforced with steel rebar gives it a higher tensile strength than cinder block. Further, it is hard to get a good read on the compression strength of a block, because you cant apply a uniform load on a hollow block.

Short answer: there are far too many variables to make a blanket statement as to whether concrete slab or cinder block foundations are stronger. Sometimes one, sometimes the other depending on variables and the particular application.

 

[John Wetter]

It's all about attachment. With all of your tornado posts, I'd really encourage you to read the EF-scale paper from Texas Tech.....

Cinder block construction joints are rated a '0' for strength when pulling apart, that's the reason why at the attachment points the bolts need to go into blocks that are filled, not just into the top block. I've seen numerous examples of floors that were lofted with the single cinder block with the bolt stall attached to the floor, but that's all that is there. A poured foundation is best. A house on grade needs to be firmly secured to the foundation, again with bolts well into the pour.

Yes, generally speaking, you need to have significant damage to commercial structures to get to the highest of the ratings, mostly due to the engineering of those structures.

 

[Greg Higgins]

Additional interesting reads:
Midwest tornadoes of May 3, 1999: Observations, recommendations, and technical guidance. FEMA 342 (didn't have time to find a quick link on the internet).

Lessons learned from the damage produced by the tornadoes of 3 May 1999 by Doswell and Brooks

http://www.cimms.ou.edu/~doswell/damage_3may99/Damage_lessons.html

 

[Angie Norris]

Additional interesting reads:
Midwest tornadoes of May 3, 1999: Observations, recommendations, and technical guidance. FEMA 342 (couldn't find a quick link on the internet).

Lessons learned from the damage produced by the tornadoes of 3 May 1999 by Doswell and Brooks

http://www.cimms.ou.edu/~doswell/damage_3may99/Damage_lessons.html

Think this may be it
http://www.fema.gov/rebuild/mat_fema342.shtm
There is also
http://www.fema.gov/library/viewRecord.do?id=1423

 

[John Robinson]

Regarding the cinder block building in Mena, which was noted in this thread and the one concerning the rating of the tornado, I think it's important to remember that none of the news coverage showed the building as it appeared immediately after the tornado struck. Keep in mind that there were approximately 17 people in that building when the tornado struck, so considerable rescue efforts (and associated rearranging of parts of the building) had taken place before the news media took pictures of the building (or the survey team saw it) the next morning. According to emergency management reports, the structure had basically collapsed down upon the seating inside the building. I'm sure the rescuers had moved things around substantially while they were working to free the people trapped inside. Damage to the residences immediately surrounding the building was substantial, making the houses uninhabitable, but mostly meriting only an EF2 rating. Furthermore, many of these houses in the neighborhood were quite old, in some cases reportedly being more than 100 years old.