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Re: Continuous Beams in Residential Cons

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AISC publishes (published) a booklet, "Moments, Shears, and Reactions for 
Continuous Highway Bridges," publication No. T106.  The title is actually a 
misnomer as the booklet has tables of influence coefficients for 2-span to 
4-span continuous beams of various length ratios.  The 2-span table goes from 
equal spans to the longer span being 1.7 times the shorter span.

The reaction coefficients are multiplied by the product of the uniform load 
per unit length and the length of the *shorter* span to obtain the reaction.

The reaction coefficient for end reaction with two equal spans uniformly 
loaded is, .3750

The reaction coefficient for the end reaction of the shorter span with the 
longer span 10 percent longer than the shorter span is, .3613

20 percent longer:  .3450
30 percent longer:  .3263
50 percent longer:  .2812
70 percent longer:  .2263

Even in this range of relative span lengths, there is only a 15 percent 
difference in the end reaction of the shorter span.

For only the longer span loaded, the uplift coefficients for the end reaction 
of the shorter span range from -.0625 (equal spans) to -.2275 (longer span 
1.7 times shorter span), still only about a 15 percent difference.


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Gary Hodgson wrote:

. > I have run into situations, particularly in house construction, where
. > continuous beams of unequal length have been 
. > used.  Where there is a long span and a short span, you usually get a 
. > kick-up (uplift) at the end of the short 
. > span.  Unless you can hold the beam end down, you get a bump in the 
. > floor. Consequently, I try to avoid these 
. > particular continuous beams.

. > I read somewhere once, that to help avoid this situation, the continuous 
. > beam spans should not vary by more than 
. > 15%.  I would like to find that piece of literature again.  Can anybody 
. > help me with this?  Unfortunately, I don't 
. > remember if it was Canadian or American.

. > Thanks,
. > Gary Hodgson, P.Eng.
. > Niagara Falls, ON

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