The standard joist tables are based on a pure uniform load. A method has been incorporated to determine if the loads on the joist are close enough to being a pure uniform load that the joist size can be selected based on a uniform load. The user specifies the tolerance used. In this method, the actual deflections and Total Load Moment and Shear are calculated based on the true loads. The maximum value of uniform or trapezoidal load for Dead, Live and Total Load anywhere along the span is then determined. The deflections obtained by applying these maximum values as uniform loads across the entire span is calculated, and the Moment and Shear obtained by applying the maximum Total Load as a uniform load across the entire span is calculated. If any of the deflections, the Moment, or the Shear thus calculated exceed the actual deflections, Moment, or Shear by more than the percent specified in the Joist Criteria by the user as Tolerance for Variation of Uniform Load, the loads will be deemed “not close enough”. Otherwise, the maximum loads thus determined will be deemed close enough, and the selection of the joist will be based on those maximum uniform loads. The user thus has control over what is considered close enough by specifying more or less stringent values of Tolerance for Variation of Uniform Load. If a value of 0% is specified, only those joists with a single, uniform load will be considered “close enough”. This method is not applicable if there are any point loads on the joist.

If the loads on a joist are not considered to be “close enough” to pure uniform, the program will then determine an equivalent uniform load based on the methodology outlined in Ref. 1. In this methodology an equivalent uniform load is calculated that would produce the same maximum moment or the same maximum shear. An equivalent uniform load is also determined that would create a shear diagram that would envelope the actual shear diagram. In calculating the shear envelope it is assumed that the end shear is at least four times the maximum shear within the middle one-fourth of the span (since the Specifications require that all members be designed for at least one-fourth of the maximum end shear). The greater of these three uniform load values is then used as the design load. If the Moment, Shear, or Deflection calculated using this design load exceeds the actual Moment, Shear, or Deflection by more than the percent specified in the Joist Criteria by the user as Tolerance for Variations of Loads, the methodology will be rejected for this joist. Otherwise, a joist size will be selected from the tables based on the equivalent uniform load.

It is important to note that when the joist selection is based on equivalent uniform loads, the design uniform load for Total Load may not merely be the sum of the design uniform load for Dead Load plus the design uniform load for Live Load. For Dead Load and for Live Load the controlling equivalent uniform load is only that which gives the equivalent deflection, since Dead Load alone and Live Load alone won’t control for moment or shear, but because the deflection criteria are different for Dead versus Live versus Total, any of those might control based on deflection. So for Dead Load and for Live Load, the design uniform load is only based on the uniform load that will produce the same deflection as the real loads. For Total Load the controlling uniform load might also be that which envelopes the moment curve or the shear diagram of the real loads. The Joist Selection report lists the controlling equivalent uniform Design Loads, and because of these differences in the determination of the equivalent uniform Dead Load, Live Load, and Total Load, the value listed for Total Load is not merely the sum of the Dead Load and Live Load values.

If any loads on the joist are in uplift, the equivalent uniform load methodologies will not be used.

If any concentrated loads exceed the maximum specified by the user in the joist criteria for Maximum Concentrated Loads, the equivalent uniform load methodologies will not be used.

If either the moment or the shears created by the concentrated loads exceeds those created by the uniform and trapezoidal loads, the equivalent uniform load methodologies will not be used.

If none of these methodologies are acceptable, the program designates the joist as a Special Joist and gives it a label of xxGP.