Maximum beam deflections are typically limited to L/360 of the beam span to prevent damage to attached materials like plaster or walls and to maintain structural integrity. Excessive deflections under service loads can compromise the durability of non-structural finishes and reduce confidence in the beam's ability to carry loads even if it is structurally sufficient. According to standard American practice, the maximum allowable deflection of a beam under service live loads is L/360, where L is the beam span.

1. 1 Prepared by Prof. Shehab Mourad – Department of Civil Eng. - KSU
Serviceability Limit State for Beams
- Deflection of beams
The deflections of beams are usually limited to certain maximum values for several
excellent reasons such as;
1- Excessive deflection may damage other materials attached to it as plasters, walls
and flooring.
2- Extreme deflection do not inspire confidence in persons using the structure,
although it may be completely safe from strength point of view.
Standard American practice for buildings has been limit service live load deflection
to approximately L/360, where "L" is the beam span.
Maximum deflection due to service live loads D max £ L /360
w
L
P
L/2 L/2
P
a a
P
L - 2a
L
L
w
P
L
D max = 5 w L4
384 E I
D max = P L3
48 E I
D max = P a ( 3 L2
– 4 a2
)
24 E I
D max = w L4
8 E I
D max = P L3
3 E I