Partial Loading

When the member is continuous, having multiple spans or an overhang, RedSpec™ considers several loading conditions and uses the worst stresses and deflections from each condition.

Uniform Loading

For example, consider a beam with two equal spans. If it is loaded uniformly along its entire length, it will bend (or deflect) the same amount on each span. The dotted line in the figure shows the deflected shape of the beam.

Alternate Span Loading

Now unbalance the beam by removing all the load from one span. The beam deflects more while loaded on one span only. Internal stresses also change when loads are unbalanced. Notice that the beam at the end of the unloaded span must be attached to the support to prevent it from rising. This is called an uplift reaction.

This is just one example of a critical loading situation. This pattern is called alternate-span loading. Another type of partial loading, adjacent-span loading, occurs on members of three or more spans. Two adjacent spans are fully loaded, while the third span is unloaded. All combinations of two fully-loaded adjacent spans are tried.

Another type of load pattern used by RedSpec is called "partial-span." It is used only on open-web truss designs, which are permitted to have only one span. By removing live load from half of the span, stresses in some of the webs may be reversed. Thus RedSpec ensures that the webs are capable of sustaining forces in tension and compression, if required. The partial-span analysis is applied on all roof trusses and on floor trusses with at least 125 psf application live load.

Designs with Holes

I-joist designs with holes in the web are subject to additional patterns. A summary of the live load configurations for a single-span I-joist with one hole is shown. For the sake of simplicity loads depicted are uniform live and concentrated live only. Dead load, which is included in all analyses, is not shown. The first four patterns are run on any floor joist with concentrated load. The last three are run if a hole is present.

For multi-span joists the partial-span configurations shown are applied to the span with the hole. Other spans are loaded with dead load only.

A round hole is shown in the figures. Load locations are indexed from the center of round holes. For square and rectangular holes, load locations are referenced from both left and right edges.

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