Is this Design Over-engineered?

As engineers, we often are accused of “overengineering” a building’s structure; when, in reality, we are just following the design rules set forth in the local building code. In most cases, contractors are set on their construction methods based on how they were taught when they first entered the industry. How many times have you heard, “I’ve been building houses for the last 30 years,” but then the local building inspector has questions as soon as the permit is submitted? What often is lost over that time period is the continuous education of the constantly updated and improved code.

As licensed design professionals, we must stay updated on the most current version of the code as well as local amendments to it. A good structural engineer is able to design the structure, while also juggling the client’s best interest regarding practical and economical design.

Here is a list of some do’s and don’ts that we’ve seen gain more attention in the most recent years:

Snow Loads (in NH and MA):

DON’T:           Assume 40lbs/ft² or use International Building/Residential Code requirements that generalize New England as the same

DO:                  Use local amendments, outlining each design load by town. Also, when designing buildings with different roof heights, snow drift from the upper roof onto the lower roof must be accounted for.

 

Hip and Valley Beams:

DON’T:           Assume it’s a nonstructural member and connect to a non-structural ridge board.

DO:                  Often live hips and valleys are considered structural members and need to be designed as beams. This means each end must be supported by another structural member and loads from the hip/valley must be transferred all the way to the foundation below.

Cathedral Ceilings:

DON’T:           Ignore outward thrust of the rafters with lack of a ceiling joist or tension tie.

DO:                  With a pitched rafter, some of the vertical roof loads are transferred in a horizontal manner, especially in areas with snow. In most cases, this is mitigated by a tension member, or a ceiling joist. In a cathedral ceiling, that member is eliminated and, to make up for that lack of structure, a structural ridge beam must be properly designed.

Structural Ridges:

DON’T:           If plans call for an LVL (or engineered wood beam) as a ridge, it does not automatically make it a structural ridge.

DO:                  Structural ridges are essentially beams sized to carry the gravity loads above. They must be posted with a load path directly to the foundation. This is similar for hip and valley beams as well.

Braced Wall Design for Wind and Seismic:

DON’T:           Ignore wind or seismic structural components because we don’t live in windy Chicago or experience California earthquakes.

DO:                  Although lower in magnitude in NH and MA, the design for wind and seismic (i.e. earthquakes) must still be accounted for. For wood framed construction, this is most practical in plywood shear walls. The length of the shear wall is often driven based on the building’s dimensions and window/door openings. Connections and fastening patterns often have specialized characteristics as well.

For further reading about the local building codes and when they are updated, check out one of our previous blogs on the International Building Code vs. the International Residential Code.