Lever Rule for Moment Distribution Factors 2

[Load Rater]

For interior girder shear distribution factors, the lever rule is recommended for structure types not found in table 4.6.2.2.3a-1 (see LRFD section 4.6.2.2.3a). Similarly, for exterior girder shear distribution factors, the lever rule is recommended for cases not addressed in Tables 4.6.2.2.3a-1 and 4.6.2.2.3b-1. However, sections 4.6.2.2.2b and 4.6.2.2.2d, which pertain to flexural moment distribution factors for interior/exterior girders, do not recommend the use of the lever rule for cases not found in the applicable tables. Is there any reason that these LRFD sections do not explicitly permit the use of the lever rule to obtain flexural moment distribution factors?

 

[wiktor]

The lever rule, although relatively accurate for shear distribution, is completely off for moment distribution, and will lead to overly conservative design

 

[Load Rater]

Thank you wiktor! In my particular case (bridge load rating with steel corrugated deck), the lever rule produces an acceptable result for exterior moment. But I was surprised to find that the relevant articles regarding moment distribution do not mention the lever rule (only for shear). Is the use of the lever rule for moment distribution factors of unique superstructure types permitted by LRFD/LRFR (even though it may be conservative)?

 

[wiktor]

Typically the lever rule is not used in the distribution of moments - the only case when it may be applicable is multi-stringer or multi-girder bridge, with relatively flexible deck and without transverse beams - some temporary bridges are configured this way. The ratio of transverse to longitudinal stiffness of the structure could be used to determine applicability.

 

[Load Rater]

Thanks again. Appreciate your time.

 

[BridgeSmith]

Table 4.6.2.2.2c-1 should be applicable for use in determining the distribution factor of the interior girders.

Though the moment distribution for exterior girders with a corrugated steel deck doesn't appear to be addressed (an oversight?), the first 2 items in Table 4.6.2.2.2d-1 would seem to envelope the stiffness of a corrugated steel deck; i.e. a wood deck is more flexible and a concrete deck is more rigid. Both specify the use of lever rule to calculate the distribution factors, so I believe it would be proper to use it for a corrugated steel deck.

Using the lever rule would be accurate for a deck with no moment continuity over the interior girders, and conservative to some degree for any continuous deck. The lever rule is essentially the worst-case scenario for the exterior girder. The stiffer the deck, the more conservative the lever rule will be.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Load Rater]

Thank you both (Wiktor and Rod Smith). We did use the table you referenced in your post for the interior girder distribution factor, and decided to use the lever rule for the exterior. This discussion has been very helpful. Thanks again.