The l stands for length.
Roof live load deflection limit.
And l 180 for unplastered roof construction.
D dead load l live load l r live roof load w wind load s snow load e earthquake load r rainwater load or ice water load.
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.
For structural roofing and siding made of formed metal sheets the total load deflection shall not exceed l 60.
You can also use af pa s span tables for joists and rafters.
D tl l 240.
Select the appropriate table in span tables for joists and rafters.
The code section on working load deflection states.
Used but they typically defined the load cases or combination stress or strength limits and deflection limits.
So if you have a beam 20ft long the allowable deflection limit is 20ftx12in ft 240 1 inch if using l 240.
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.
These tables are from the icc international residential code.
If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6.
L 240 for unplastered floor construction.
This sets an allowable first floor live load of 40 psf a dead load of 10 psf and a deflection of l 360.
Table r301 7 allowable deflection of structural members.
For secondary roof structural members supporting formed metal roofing the live load deflection shall not exceed l 150.
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.
So these are the limits set by the code.
In the absence of more specific criteria criteria for structures with brittle finishes as found in code documents for years is frequently used.
For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.
The table of contents indicates that table f 2 matches these loading conditions.
Figure 2 live loads and deflection limits are set by code.
D ll l 360.
The deflection of floor and roof assemblies shall not be greater than l 360 for plastered construction.
These limit states are mathematic expressed as.
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.
For secondary wall members supporting formed metal siding the design wind load deflection shall not exceed l 90.
These limits were originally developed for members with brittle.
