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International Building Code vs International Residential Code

Code ApprovedThere are 2 codes primarily followed by most code officials. The International Building Code (IBC) is the main code used for most construction. Although latitude is given to the engineer of record for a project for designing a project and taking responsibility for its designed performance, the code gives guidance for engineers and code officials with standards and protocols for accepting products and their designed use. The general format is for soils investigations to inform the design engineer for their use of foundation design to interact with those soils. The process requires calculations to be performed to give evidence of a proper design. Those calculations are checked by the code official.

The International Residential Code (IRC) is meant for use specifically in residential applications where in lieu of specific engineered designs a standardized prescriptive designs are followed. The prescriptive designs are not informed by investigation. Instead their prescriptive designs are done more conservatively to compensate for the lack of a rigorous design process.  Instead of using calculations, a preset prescriptive design is used. The conservatism is usually accomplished with using higher safety factors.

An example is the use of assumed minimum load bearing capacity for soils. The IRC gives guidance of using 1500 pounds per square foot without soil information. Cities can then allow this assumption (or another one if they choose) unless a soil investigation is performed to show higher bearing capacity. The code official if he has evidence based reason to believe that a particular soil cannot bear 1500 lbs per square foot, can reject the assumptive minimum bearing capacity and require a soil investigation.

The process of underpinning design for foundation repair has been a prescriptive from the very beginning of their use. Lately the IBC has attempted to bring this process in more into the engineering design process. While the attempt is applaudable, there has been some unintended consequences that have put undue burden on home owners with little benefit for them.  

Requiring soil boring data to design piles for underpinning is expensive and provides little benefit as the soil data has a negligible effect on the pile design for residential structures. Thus a prescriptive approach makes the most sense.

Recently  IAPMO, one of the organizations that provide guidance to code officials, has published EC 027. This publication, while not perfect, does recognize the prescriptive nature of helical piles and give guidance for their use without soil information. Code officials should welcome this guidance.

In my next blog, I will go into more details of what this consists of and have additional suggestions to aid code officials, particularly the MAG (Maricopa Association of Governments) who make recommendations for code officials in Maricopa County.

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