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In the AISC publication, only Tables 2.0 through 4.7 consider constant moment of
inertia. These tables
show the actual force effects.

I use the ordinates in Tables A2.0 through A4.7 as well as those in Tables 2S,3S
and 4S for a quick and
inexpensive means to calculate moving live load moments, shears and reactions.

BTW, an increase in moment of inertia of 50% of the girder over the interior
supports gives approximately
a 5% increase in negative bending. Since Chris didn't elaborate the details, I
kept my response generic.

There are several issues to deal with when proportioning continuous girders. We
can discuss privately,
you know my number.


hmorera(--nospam--at) on 09/30/99 02:17:50 PM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)
cc:    (bcc: Tom Cummings/NewYork)

Subject:  Re: AASHTO ASD/LFD


Please note that those tables in the AISC publication (or at least the version
of the book that I have) assume constant moment of inertia over the entire
length of the structure.  Increasing the stiffness at interior piers will tend
to increase the negative moment at that location and reduce the positive moment
at midspan.  The reason for varying the moment of inertia over the length of the
structure is evident when you see the moment diagram which allows for economy in
member sizing.  But for initial girder sizing that publication is very useful.
Sorry, I don't have an inexpensive program to suggest to you.  Check Penn DOT's
website.  I think they have programs for $500.  Though, I think they think they
went to LRFD.


Tom_Cummings(--nospam--at) on 09/30/99 11:44:10 AM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)
cc:    (bcc: Hector Morera/NYC/AmmannWhitney)
Fax to:
Subject:  Re: AASHTO ASD/LFD


For quick and inexpensive, why not just use influence ordinates?  They are
available in AISC publication  titled "Moments, Shears and Reactions for Highway
Bridges" as well as in a book by  Georg Anger, titled "Ten Division Influence
for Continuous Beams".

If your span ratios are non-symmetric, a book by Boyer and Abrams titled
"Influence Lines for Continuous Beams" will be helpful.


Tom Cummings

"Chris Towne" <ctowne(--nospam--at)> on 09/30/99 10:41:10 AM

Please respond to seaint(--nospam--at)

To:   "SEAINT List Server" <seaint(--nospam--at)>
cc:    (bcc: Tom Cummings/NewYork)


Does any body know about a cheap live load generator out there?  Basically I
just need reactions for a single lane load on a continuous bridge.

Chris Towne, E.I.
Chapman Technical Group
St. Albans, WV