Design Spans and Iterative Design

Design Span Input

Sizer provides the option of entering

• Design span, extending from center of interior supports to a distance 1/2 the minimum required bearing length past the inner edge of an end support. For single span members it is the distance between the points that are 1/2 the minimum required bearing length from the inner edge of the supports.
• Full span, extending from the outer edge of exterior supports to the centre of interior supports. For single span members, it is the length of the beam (the projected length for sloped members).
• Clear span, extending between the inner edges of supports.

Span Drawing

Regardless of the input choice, each of the span type dimensions is shown in the beam diagram, with the word (unknown) showing if the dimension cannot be established until design.

Interaction with Bearing Design

For each of the span types, the span used for design calculations is not known until either or both of the bearing length and the minimum required bearing length is established.

Unless Design span is chosen as the span choice, the calculation of minimum bearing length affects the design span length. This in turn affects the reactions that on the supports, which in turn affects the min required bearing length, which in turn affects the design span...

Similarly, if unknown bearing length is selected, the reactions determined affect the bearing length required, which in turn affect the position of the edge of the support from which design span is measured, which affects the design span, which affects the reactions used for bearing design…

Iterative Design

For these reasons, the program performs iterative analysis and bearing design on the member, first determining reactions with the longest span possible given the bearing and span choices, then determining the bearing lengths required, then calculating design span, then readjusting. The program iterates until the difference in design span between two iterations is not noticeable (0.1 mm). Generally speaking 3-4 iterations are required

Use minimum bearing to determine design span setting

You may want the program not to design a bearing less than some practical amount, say 3.5”, however you may also wish to take advantage of the reduced design span by including only the minimum required bearing length. For his reason, you can opt out of using the minimum bearing entered in the Design settings in your calculation of design span. The program in this case uses the minimum required bearing for design span used for shear, bending and deflection, but the value entered in the settings for bearing design.