Positive Loads, Negative Loads, and Skip Loading
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 some codes, when a beam is cantilevered at one or both ends RAM Steel Beam 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 skip load the Live loads, it applies the Positive and Negative loads on alternating spans such that the worst conditions are calculated.
- Positive Live Loads on Left Cantilever, and Negative Live Loads on Span and Right Cantilever
- Positive Live Loads on Left Cantilever and Span, and Negative Live Loads on Right Cantilever
- Negative Live Loads on Left Cantilever, and Positive Live Loads on Span and Right Cantilever
- Negative Live Loads on Left Cantilever and Span, and Positive Live Loads on Right Cantilever
If there are no Negative Live Loads, a value of 0.0 is applied to the particular cantilever or span in the above conditions. Similarly for Positive Live Loads.
Roof Live Loads are also skip loaded.
For the precomposite condition of unshored composite beams the Construction Live Loads are also skip loaded, similarly as for Live Loads, except both Positive and Negative Construction Live Loads are applied simultaneously.
Dead load is not skip loaded; the full dead load is applied to all spans and cantilevers.
Where BS 5950 and Eurocode require the investigation of the interaction of moment and shear effects, each unbraced segment of the beam is checked for the maximum shear (with associated moment) and the maximum moment (with associated shear) using the skip loading conditions which produce the maximum moment, and for the maximum shear (with associated moment) using the skip loading conditions which produce the maximum shear.
RAM Frame does not skip load the Live loads except when calculating the reactions on the frame members from gravity members with cantilevers.