Over Design, Dome and Ring beam construction
By: Hubert Crowell
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.
All rights reserved. No part of this work covered by the copyrights hereon may be reproduced or copied in any form or by any means-graphic, electronic, taping, or informational and retrieval systems - without the written permission of the author.