These limits were originally developed for members with brittle.
Roof live load deflection limit.
In the absence of more specific criteria criteria for structures with brittle finishes as found in code documents for years is frequently used.
And l 180 for unplastered roof construction.
Used but they typically defined the load cases or combination stress or strength limits and deflection limits.
D tl l 240.
A single concentrated live load l of 3 000 pounds 13 35 kn applied over an area of 4 5 inches by 4 5 inches 114 mm by 114 mm and located so as to produce the maximum load effectson the structural elementsunder consideration.
This simplistic criteria puts a limit of the span divided by 360 on the incremental deflection due to live or transient load only and a limit of the span divided by 240 on deflection under total load.
On a roof with a slope greater than 4 to 12 the live load limit is typically adjusted downward from 20 psf to 15 psf to allow for the relatively greater dead load on the steeper roof.
Table r301 7 allowable deflection of structural members.
You can also use af pa s span tables for joists and rafters.
If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6.
The l stands for length.
So if you have a beam 20ft long the allowable deflection limit is 20ftx12in ft 240 1 inch if using l 240.
For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.
The deflection of floor and roof assemblies shall not be greater than l 360 for plastered construction.
This means that your actual calculated deflection of the beam should not exceed either 1 inch or 0 67 inch depending on which of the criteria you are using.
For structural roofing and siding made of formed metal sheets the total load deflection shall not exceed l 60.
These limit states are mathematic expressed as.
Or 20ftx12in ft 360 0 67 inch if using l 360.
Step 2 span table.
Select the appropriate table in span tables for joists and rafters.
So these are the limits set by the code.
The table of contents indicates that table f 2 matches these loading conditions.
For secondary roof structural members supporting formed metal roofing the live load deflection shall not exceed l 150.
D ll l 360.
These tables are from the icc international residential code.
Load types loads used in design load equations are given letters by type.
L 240 for unplastered floor construction.
Figure 2 live loads and deflection limits are set by code.
The code section on working load deflection states.
