In the mid 1960’s I was working with Post Tension concrete projects in the southeast United States for Noble Concrete, Inc. Many of our large projects would be stressing ring beams for dome concrete buildings. Domes exert a large lateral load on the walls of the buildings upon which they set. The ring beam is what holds the walls from falling outward from the weight of the dome.
In concrete construction there is usually a large safety factor built into the construction in the way of extra steel bars. Post Tension concrete is a little different, too much of a good thing can cause trouble.
The design engineer will note on the plans how much tension is required and suppliers like Noble Concrete would bid on the jobs and then make the shop drawings showing the placement and size of the cables required.
We got the contract of a church dome in Jacksonville Florida and all was going well. When the concrete had been poured and reached the proper strength I loaded up the stressing equipment and headed south. Our time on the construction site for a job like this is usually no more than a couple of days and this one could be completed in less than a day. Of course there is always the problem with the Unions. They insist on having one of their people do the work. However, they never have anyone trained in this type of work. We give them a call and they send us a couple of workers, we tell them just to stand there and watch and we complete the job a fast as possible.
We were making our second trip around the ring beam bringing the stress up in stages to prevent damage to the columns. I had planned on connecting to each cable four times during the stressing procedure. Suddenly there was a large pop and all that I could see were workers scrambling down off the building!
We stopped the stressing and inspected the columns for cracks. I also climbed to the top of the support for the dome and checked the top of the form. We had raised the dome about one inch off the top of the form. The columns were pulled into the point that one of them had cracked!I got on the phone to the engineer and asked him what we should do next. His reply was “is it holding up the dome? Then your job is through.” He had over designed the ring beam just as he over designs when using steel bars and if he was aware of what would happen, which I doubt, he did not warn us. We relayed the information to the contractor and packed up for home.
About the Author: Hubert Crowell, Cave Explorer
I have started writing as a hobby and plan to write about my life, work, hobbies, religion and many other things of interest to me and maybe others will enjoy also.
For more information on caving, improving your service department and many other subjects, Please visit my web site at:
http://hubertcrowell.name/
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