In the design of steel beams with cantilever extensions, the program assumes that live loads on the cantilever may or may not occur along with those on the back-span. This is true for all types of live loads including roof live and snow loads. Dead loads including self-weight are always acting on the full span.
On the output, loads are referred to as Positive or Negative. A Positive load is a downward acting load while a Negative load is an upward acting load. For simple span beams Positive loads create beam reactions that become Positive loads on the supporting members. However, Positive loads on a cantilever create uplift on the backspan support, the reaction of which becomes a Negative load on the supporting member. Negative loads only occur as a result of cantilevered beams or from user defined negative loads. The program keeps track of Positive and Negative loads independently.
On the output, moments and reactions are labeled Maximum Positive and Maximum Negative. If there is no Negative value, it is listed as 0.0 or not shown at all. If there is no Positive value, it is listed as 0.0 or not shown.
As required by Code, when a beam is cantilevered at one or both ends RAM Steel Beam Design skip loads the Live load on adjacent spans and alternating spans such as to create the maximum moments, deflections, and reactions. Not only does RAM Steel Beam Design skip load the Live loads, it applies the Positive and Negative loads on alternating spans such that the worst conditions are calculated.
The following skip load condition is performed to calculate the maximum positive moment (top flange in compression):
The following skip load condition is performed to calculate the maximum negative moment (bottom flange in compression):
The following skip load conditions are performed to calculate the maximum shear force:
To determine the maximum support reactions, the program uses a similar approach. The reported reaction is the total reaction at the support. For example, this image below shows the live load pattern to determine the max reaction at the left support for a double cantilever condition:
To determine the critical shear reaction on either side of the support it is recommended to use the shear envelope diagram.
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