Lateral Support Changes

Previously, Sizer evaluated the K_L factor for a particular span by determining the unsupported l_u in the vicinity of the maximum negative and maximum positive moment points for that span. In doing so, it

• included points of zero moment in mid-span as being equivalent to lateral support points,
• did not include bearing support points if you had input a value for intermediate lateral supports,
• Considered the small remainder when lateral supports divided unevenly into a span as being the l_u for that region of the span.
• did not reflect this situation in the drawing of the beam

Now, the program uses the smallest of the beam span and the user-input lateral support spacing as the l_u in all cases, and the drawing better reflects a typical configuration.

The components of this problem are discussed in more detail below, along with some smaller issues that have been addressed:

Point of Zero Moment for Unbraced Length for Steel Beams (v8.4- Bug 2686)

The calculation for unbraced length for steel beams was considering a point of zero moment to be equivalent to a lateral support, as the program had been doing for wood members. However, the commentary for section 13.6 of the CSA S16-09, in the Handbook of Steel Construction, states that “Points of contraflexure for bending about the major axis are not related to lateral-torsional buckling and therefore cannot be considered as points of lateral support (Schmitke and Kennedy 1985).”

As a result, the moment resistance for multi-span and cantilevered beams without additional lateral support was non-conservatively based on a span that was too short. This has been corrected, and points along the beam where the moment transitions between positive and negative values are no longer considered start and end points in the calculation of unbraced length.

Point of Zero Moment for Unsupported Length for Wood Beams (v8.4- Bug 2695)

The program had been using the point of zero moment in multi-span beams as if it were a point of lateral support when determining the unsupported length lu for the KL factor on CSA O86 6.5.6.4.1. This is because the design code clause refers to lateral support of the compressive edge, and the point of zero moment marks the start of the compressive region of the beam edge. For the following reasons, the points along the beam where the moment transitions between positive and negative values are no longer considered start and end points in the calculation of unsupported length:

A literal reading of CSA O86 6.4.6.4.1 allows this only for the case where intermediate support is provided, not in the case where there is no additional lateral support between bearing supports.

When there is additional lateral support between bearing supports, 6.4.6.4.1 says to use the maximum purlin spacing, so an interval with a full spacing rather than one containing the point of zero moment should be used.

For purlin spacing that divides unevenly into the beam length, it is always possible to position the supports such that the longest lateral support interval does not contain a zero-moment point, and since the positioning is arbitrary, the most conservative configuration should be used.

The reasoning in the Schmitke and Kennedy (1985) paper cited in the steel design commentary (see Bug 2686 above) applies to wood as well as steel.

Because the program was also evaluating l_u in the vicinity of the maximum moment (see bug 2708), this frequently resulted in smaller than expected l_u values and higher than expected K_L.

Unsupported Length for Multi-span Members with Intermediate Support. (v8.4- Bug 2700)

For a multi-span beam, when you entered a value for intermediate lateral support, the program did not include interior supports as points of lateral support. However, CSA 086 6.5.6.4.1 says to use the maximum purlin spacing, but only in the context of that spacing being “intermediate” to a span, and the intention is not to use the purlin spacing when it is greater than the span length.

This created longer than expected lateral support distances for maximum negative support moments at supports, and a lower than expected K_L value. It has been corrected, and the program always includes bearing support points as points of lateral support. Note that the program has always indicated via a design note that these points are to be restrained.

Lateral Support Evaluated at Point of Maximum Moment (v8.4- Bug 2708)

The program was determining the lateral support interval to be used in calculating the K_L factor for CSA 086 6.5.6.4.1 as being the one in the vicinity of the point of maximum moment in a span, so that if lateral supports divided unevenly into span length, and the maximum moment lay within the small remainder distance, that distance would be used as the unsupported length l_u rather than the full user input span length.

CSA O86 6.5.6.4.1 says to use the maximum purlin spacing, so the program has been corrected to use the user input spacing value.

Note that the program would rarely if ever use this small distance, which is positioned at the right end of a span, as the l_u , because the maximum positive moment usually occurs at mid-span. The maximum negative moment is usually at a support, in which case the full support distance in the span to the right of the support governs.

However, the use of this method was exacerbating the problems created by bug 2695 (above), in that small lateral support distances created by including the points of zero moments were often in the vicinity of maximum negative moment.

Drawing of Lateral Support (v8.4- Bug 2697)

The drawing of the supports was such that the first support was always placed at one half the input lateral support spacing from the end of the beam.. This created a confusing situation when resulting support is the only one drawn, and at other times resulted in more supports than are necessary. Also the positioning of the support used to calculate the unsupported length l_u was based on starting the lateral support spacing at the start of the beam, so was not reflected by what was shown on the screen.

The program now draws the first lateral support at the support point for the top of the beam, to reflect the fact that it is to be laterally restrained at the top. At the bottom, it draws the first support at a distance equal to the lateral support away from the bearing support point. It does so independently for each span.

Design Code Clause for Lateral Support Note (v8.4- Bug 2706)

The design note in the Results output about the need for lateral restraint at bearing support points references the sawn lumber clause 5.5.4.2.1 for all materials. It now references 6.5.6.3.1 for glulam and does not reference any clause for steel design.

Lateral Support Note for Glulam (v8.4- Bug 2706)

The design note in the Results output about the need for lateral restraint at bearing support points appears for glulam members only when the d/b ratio is greater than 2.5:1 as per 6.5.6.3.1.

Materials in Input Note for Lateral Support (v8.4- Bug 2707)

The note appearing below the lateral support input in Beam View now appears for sawn lumber and SCL only, and no longer for glulam, I-joists, or steel. This note refers to clause 5.5.4.2.1 for sawn, which is also referenced for SCL materials. The Design Setting that controls this note and the engineering functionality apply to sawn and SCL only.

Ratio in Input Note for Lateral Support (v8.4- Change 143)

The note indicating that the lateral support inputs apply only when d/b is greater than 9 was saying "b/d" rather than d/b. This has been corrected.