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Structural Investigation: Options

Every modern building code I’ve seen has, somewhere, some opt-out language. Not for opting out of the code, as that would mean that the code was unenforceable, but rather for opting out of specific requirements. For example, in the 2015 IBC:

104.11 Alternative Materials, Design and Methods of Construction and Equipment

The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code in quality, strength, effectiveness, fire-resistance, durability and safety. Where the alternative material, design or method of construction is not approved, the building official shall respond in writing, stating the reasons why the alternative was not approved.

Taken at face value, you can use any material or any design and analysis method as long as you can demonstrate to the satisfaction of the local building officials that what you propose is as good as the more usual material, design method, or analysis method that is specified in the code. Assuming a minimum amount of good will on the part of the building official, you need engineering back-up for what you want to do (calculations, material data from an ASTM test, or similar demonstration of a rational method) and the strength of your convictions in presenting it.

What does this have to do with building investigation, as the title of this blog post suggests? Simple: a structural investigation depends on a model or models of the structural action of the building. You can’t come to any meaningful conclusions without a model that gives you some idea of the magnitude of loads, the load path, and the form of the structural members that make up that load path. I mentioned the Lincoln Building on Union Square a few days ago. Its structural model consists of terra-cotta tile arch floors (compression vaults), carrying load to iron floor beams and girders (doubly-symmetric ductile beams in bending), carrying the load to cast iron columns and brick masonry walls (brittle and generally stocky compression members), carrying the load to masonry foundations. That’s a common model for nineteenth-century commercial buildings in New York. But sometimes we’re faced with an old building where the common models don’t work. The manner in which the building is currently carrying load may be non-standard and may be something we would not build today and that is okay. The question we have to ask is not whether it meets the specific requirements of today’s code (it almost certainly does not) or the code in force when it was built (it might or it might not). It’s whether we can show that the way it works is equivalent to what it was meant to be. This is both more difficult than simply looking for code compliance and much more interesting.