Building Challenges: Deflection of Beams
California leads the trend of indoor / outdoor living, which translates to massive openings, greater window space, and wider doors; paired with a desire to maximize openings and avoid headers, modern houses are increasingly designed with floor to ceiling windows and doors. Designs without any elements between the actual structure creates a demand for the use of steel or beams; with this demand comes an even greater challenge of managing the deflection of beams during the course of construction.
What happens when a beam deflects?
The degree to which a beam is displaced under a dead load or gravity is called deflection. When a beam deflects over a rigid window or door unit this closes the nominal “gap”, or room, between the unit and the structure causing for sticking, breakage of the glazing, and worst case collapse. Without the proper calculation from engineers or the foresight to mitigate the deflection before it occurs, this can be detrimental to a build in the later stages of construction.
As general contractors, we rely heavily on information disseminated from the multitude of parties involved in the design of a home, as well as the ability to foresee missing information and possible challenges. We have seen deflections from ¼” to 1 ½”, which when working on modern homes with little to no room for deflection, the slightest movement may as well be a mile.
How to avoid deflections
Structural plans show only the structure and not the allowable deflections. In many instances, plans are designed with allowable deflections, however the information is not openly transmitted to the contractor. Always ask the structural engineer for the deflection table on framing members – especially at floor to ceiling units where there is a nominal amount of play. Contractors should always take the deflections into account when installing all floor to ceiling units. Additionally, consider a small drop ceiling; a 2” minimum can allow for the “straightening” of the structural members once the loads have settled.
Material selection is equally as important. Glulam and Parallam beams have become the default choice for builders and architects. With Glulam beams serving best in applications with more expansive load sections, it is a versatile product available from straight to curved members. Parallam has high design values and work better in smaller sections. As Parallam beams get longer, the taller and wider the beams need to be, creating issues in installing and routing things like plumbing and HVAC.
Another option includes kiln-dried wood and other types of wood and composite materials. Kiln-dried lumber as proven to shrink and twist less making its movement more predictable. Of course, if the budget allows, steel is always the preferred, as it presents the most tolerance to deflection and can be “boxed out” post installation to achieve the desired aesthetic.
Changes in various metal and woods suggest that engineers and builders alike continue to look for better ways to build open space structures using more than just structural equations. Materials continue to move to the forefront of the ever changing building challenge.
For answers to building solutions, please visit Boswell Construction at http://www.buildboswell.com.