Remodeling Existing Structures

When designing a new structure, every detail of the building has either been specified by the architect or is able to be decided by the engineer.  However, when renovating an existing structure, analysis can be significantly more complicated.  First, any important details of the structure need to site verified rather than relying entirely on As-Built plans because often the a structure will not exactly match the plans due to either changes during construction or modifications made to the structure.  Furthermore, there will be cases, especially in very old structures, where no plans are available.  In some structures, such as steel structures, architectural features may be pulled down to expose the structural elements allowing a determination of the members and connections forming the structural system.  However, in the case of concrete structures, size and location of rebar reinforcement cannot be determined without non-destructive testing methods (e.g. Ultrasonic testing).  This type of testing can be quite expensive, especially if it is required for the entire structure.

Last week, a group of our students toured a 2-story concrete building in downtown Fresno that is being remodeled by Bitwise.  This building was constructed in 1925 as a raisin packing house and now building plans for the structure exist.  However, in lieu of implementing costly non-destructive testing to fully analyze the structure according to current codes, Barcus Engineering (the structural engineering firm working on the remodel) invoked a particularly useful section of the CBC, i.e. Chapter 34A - Existing Structures, which states that:

          1.  Gravity supporting members for which loads are not increase by more than five percent can be presumed to be adequate without strengthening.
          2.  Existing lateral load resisting elements for which the demand to capacity ratio does not increase by more than ten percent may remain unaltered.

The first of these can be determined by comparing the proposed additional load to the existing loads supported by the structure.  It is important to consider not only additional dead loads, but also additional live loads caused by changes of use.  For the second, additional seismic lateral loads are directly related to the additional seismic mass proposed to be added to the structure.  Reductions in capacity may be assessed in terms of the percentage of the system removed.  E.g. removing a 5 shear wall along a wall line with 50' of shear wall would indicate a 10% reduction in capacity along that wall line.  These guidelines are based on the understanding that the factor of safety on the original structural design must have included enough conservatism to allow for these modest increases in demand.    This portion of the building code can help significantly in making remodeling of historic structures, such as the one Bitwise is remodeling, feasible.

Students touring the second floor of the Bitwise building.  Note the original roofing system with cross blocking instead of full depth blocking.

Touring the first floor of the Bitwise building. Note the custom shaped structural steel that has been installed along the columns and ceiling to reinforce the concrete structure for the new gravity loads.