Messages and Online Forum Q&A

Re: Home Extension

From: info@builderswebsource.com
Category: Structural
Remote Name: 63.198.181.43
Date: 03 Feb 2002
Time: 11:47 AM

Comments

Your question is too general to provide specific structural recommendations. However, here are some general guidelines to follow in a situation such as yours:

First, Builders Websource always urges our readers to retain the services of a local state-licensed structural engineer, particularly when it comes to complex load calculations for additions and remodels. The problem is that every home is unique and the "as built" conditions can only be determined by on-site inspection.

In any structural engineering exercise, you must know exactly how the loads of the existing and new structures will be transferred to load-bearing walls. Many factors influence this, including the area, height and type of materials used, notation of any point loads (such from roofs), snow, wind and seismic loads, as well as the condition of the existing foundation and soils.

Together, these factors make up a complex interplay of structural components, each of which must be considered when modifying a structure. In addition, other factors also come into play, such as "acceptable" floor stiffness...which is often a subjective measure. For example, while the building code allows a floor to deflect up to its clear span (in inches) divided by 240, this means that the total deflection will increase as the span increases. However, on longer unsupported spans, such as 15-20 feet or more, this could result in a deflection of nearly 1" under fully loaded conditions! While this meets building code, a 1" sag in the middle of the floor may not meet your expectations for a quality structure. Therefore, it is recommended that deflection be limited to no more than about 0.3 inches, or less. In the above case, this would require designing the structural members to nearly 3 times the minimum required by the building code!

Furthermore, if you have an older home, such as you describe, additional reinforcement may be required in the brick walls to bolster its lateral or shear strength. While masonry is generally quite strong under compression, removing portions of your wall will weaken the structure. Sometimes wood or steel column supports will need to be added to support point loads.

These tips are general, but provide insight into why it pays to hire an expert who can properly analyze your site conditions. The good news is that almost anything is possible to overcome, so long as you obey the fundamental principles of engineering, coupled with enough money to have it built right by a competent contractor.

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