Floor Deflection Limits

Clear span in inches l over a given number.

Floor deflection limits. Larger of twice the span deflection criteria or 0 2. So if you have a beam 20ft long the allowable deflection limit is 20ftx12in ft 240 1 inch if using l 240. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection.

Unfortunately however code compliance does not automatically equal customer satisfaction. 400 mm the maximum deflection that can be allowed when using l 360 is 16 360 which equals 1 25 04 or 1 mm. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.

They are expressed as a fraction. See the table below. Short cantilever deflection limit.

Builders design for deflection only typically by holding deflection due to live load to a maximum of l 360 where l is the floor joist span or perhaps a more restrictive l 480. Cantilever deflection limits are twice the span deflection criteria for both live load and total load deflection. For example a floor girder spanning 36 ft may deflect up to 1 2 inches under a live load only deflection limit of l 360.

Any non structural partition under the beam must be able to accommodate this deflection. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. In practical terms this means in a residential application with joists 16 o c.

The l 360 standard means that the floor should not deflect more than the span divided by 360. For example a floor with a span of 10 feet must deflect no more than 1 3 inch or 120 inches 360 inches. Or 20ftx12in ft 360 0 67 inch if using l 360.