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RE: BRIDGE DESIGN: Questions re Live Load Distribution Factors in AASHTO-LRFD

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Bill:

Since no one has touched this one yet, I'll take a stab.  I have not had the
opportunity to use the LRFD Specs. for design, but I am familiar with them.
For both skewed and curved bridges, my understanding is that the LRFD treats
them much the same as previous versions of the AASHTO Specs. That is, when
skew or curvature is below some arbitrary limit it can be ignored, or dealt
with empirically (skew factors for moment and shear, LRFD 4.6.2.2.2e and
4.6.2.2.3c).  Beyond these limits, the curvature or skew must be dealt with
analytically, by grillage, finite elements, etc.

Skew:   Since skew affects both moments and shears, reducing them in some
locations and raising them in others, a "line girder" analysis with constant
distribution factors will overestimate moments and shears at some locations
while underestimating them at others.  Both CA and Nevada provide empirical
guidance for estimating these variations, but for very highly skewed
bridges, this may not be adequate.

Variable Spacing:	A good approximation of moments and shears for
non-parallel girders can be achieved by varying the distribution factors
along the length of the girder.

Curvature:	Moments and shears in girders of curved bridges vary
significantly (higher on the outside girders), lower on the inside girders,
and near the average at the center.  The only way that I am aware to
approximate the affects of curvature is the "V-load" method.  This is a
modified "line girder" analysis (for steel girders) that approximates the
torsional affects on the group of girders by adding lateral flange moments
to those calculated for a straight girder.  There is a good discussion of
this method in the "Highway Structures Design Handbook" published by AISC
Marketing.  Otherwise, there are the grillage and finite elements methods.
Incidentally, for steel bridges, another reason that standard distribution
factors are not applicable is that there are significant forces carried by
the cross frames, which thus must be designed as primary members.  Finally,
if I'm not mistaken, the AASHTO Guide Specification for Design of
Horizontally Curved Steel Girders (steel again!, you'd think I worked
somewhere other than California) still applies with the LRFD Specs.

The simple answer to your questions:  Yes, in some cases.

Is that any help, or did I just tell you everything you knew already?

Tony Powers
HDR Engineering


> -----Original Message-----
> From:	Bill Polhemus [SMTP:polhemus(--nospam--at)insync.net]
> Sent:	Wednesday, June 10, 1998 1:59 PM
> To:	'seaoc(--nospam--at)seaoc.org'
> Subject:	BRIDGE DESIGN: Questions re Live Load Distribution Factors
> in AASHTO-LRFD
> 
> In studying some of the background material from development of this
> portion of the LRFD Spec (notably regarding the NCHRP Project 12-26 which
> provided much of the basis), I have come to realize that the methods used
> in the older versions of AASHTO had oversimplified this procedure,
> sometimes detrimentally.
> 
> The new procedures, while they are much more involved, also seem to have a
> larger confidence factor attached.
> 
> However, I notice that among the "limitations" of the simplified equations
> shown in Article 4.6.2.2 of the Spec, it is mentioned that you have "beams
> which are parallel and approximately of the same stiffness," and "have
> in-plan curvatures less than" a certain limiting amount."
> 
> I would like to know what any of you, experienced with this Spec, think
> about the implications for a very common geometry: a bridge with skewed
> supports and a roadway that is curved in plan.
> 
> We have a lot of these, and so of course the beams will in most instances
> not be parallel.  And the curvatures can be pronounced (although not
> necessarily always as much as the Spec limit).
> 
> Does this mean I need to use a grillage model, or "worse," just to come up
> with the moments in these girders?
> 
> Your comments are very welcome.
>