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RE: Existing Buildings

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Based on my experience, the local building inspector will need to approve the method of qualifying the structure.  There are different methods to qualify the structure that include analysis and load test.  If the structure was built in the 1900s, there were building codes governing the design of the structure.  As a minimum, we use the building code in effect when the building was constructed to qualify by analysis.  ACI 318 permits load tests and we have used load tests to qualify the structure.  
IBC has a building code for existing buildings.  Also, see ASCE 11-99, Guideline for Structural Condition Assessment of Existing Buildings. 
Concrete cores and rebar testing to determine material strengths are used in the analysis to qualify the structure.
We do not disregard the building codes.  The local building inspector will probably not allow this either.  Our insurance carrier would probably not provide coverage if we neglected industry codes and standards.  Disregard of codes is not in the best interest of the public - in my opinion.
Gary Loomis, PE
We have done some additions to existing buildings and checked adequacy of floors for increases in loadings many times before. But I always checked the existing building's original structural drawings against current codes when making decisions, along with conducting some site visits for some verification of construction. 
Today I got into a discussion with an engineer that has done a lot of rehab work. He states that since there is no existing building code, you can use whatever analysis and design methods you choose (obviously you should still meet statics) and that no codes apply. The only provisions in the IBC are in chapter 34 which he said has a provision that allows you to completely diregard the building codes and standards that are intended for new buildings when looking into the capacity of an existing building. 
For instance, if there is an existing flat plate reinforced concrete slab that is being analyzed to carry up to 50% more capacity than it was originally designed for under ACI 318. If enough cores are taken to justify the actual concrete strength, the existing reinforcing strength is  known, and the locations of all the rebar are verified you can completely disregard ACI 318 and use whatever redistribution of moments is required to get the slab to work, disregard all ACI's phi factors, and use tension rings or whatever other method you can come up with to show that the slab works. 
Does anyone here have experience in checking the capacity of existing designs in this manner, with total disregard to the building codes? Is this typical practice when substantial verifications of the existing construction can be accomplished? 
Will Haynes, P.E.