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This feature allows you to specify the configuration of the floor or roof area that is assumed to create an area load that is entered on a beam, so that the load from that area load is distributed to the supports via engineering mechanics rather than simple tributary width.
User Interface
The program previously allowed for a checkbox in the load view only indicating that the beam supported an area load from continuous members, so that the resulting line load is 1.25 times what it would be if tributary width was assumed, as for joists that are broken at the support.
Now you can select 2 spans for the supported joists, or some other configuration. If two spans is selected, then you enter the ratio of the span lengths, and the program calculates the percent of the area load that is used to create the line load on the beam, and shows that value in the grayed out percentage input.
If “Other” is selected, the program enables the % of area load on the beam control and then you enter the percentage.
Load Diagram
Previously, the conversion to the line load from the area load was not reflected in the diagram, which showed the resulting line load as if it was due to tributary width. It is now shown as the one converted using the engineering mechanics of the selected configuration, rather than tributary width.
Text Output
The note that appears in the loads table has been updated to indicate the percentage of area load applied.
Engineering Design
Previously, the checkbox was checked, when area loads were converted to line loads, the conversion was area load intensity x tributary width x 1.25, which is twice the proportion of load taken by the centre beam support of a joist with two equal spans ( noting that the tributary width is half the full length of the supported joist. )
In order to apportion the load to the centre span for uneven spans, it was necessary to express the proportion of the reaction on the centre span in terms of the ratio of the length of the two spans, which we will term r.
The equations for the circumstance we are modelling are found in NDS Design Aid 6, Beam Design Formulas with Shear and Moment Diagrams. These equations were adjusted to show the center reaction R2 as a ratio of the two spans:
R2 = (r3 + 4r2 + 4r + 1 ) / ( 8r (r+1) )
The line load intensity is then 2 x R2, x tributary width x area load intensity, noting that R2 is the proportion of the load on the full length that is taken up by the support, and the tributary width is half that length.