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Re: AASHTO Lane Loads

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Ed,

Roger pretty much answered the question but gotta add that lane loading is
placed completely on a span and arranged on a span by span basis to maximize the
forces you need (assuming of course that lane load, not wheel loads govern).
For example, if you have a 4 span continuous structure with every span being the
SAME LENGTH, maximum moment and shear will be produced at the first interior
support if you load spans 1, 2 and 4 completely (see AISC beam tables).  To
maximize shear at one end support and positive moment at one of the end spans,
place the load over spans 1 and 3.  Just keep doing this until to maximize all
of the critical locations.  It gets to be more fun when you have significantly
different lengths between spans.

Hope this helps.

Hector






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Subject:  AASHTO Lane Loads




Ed,

The figures that I am referring to are in the 15th Edition of the AASHTO
Bridge Specs but should be the same or close to the 16th edition --- I don't
have a more recent version (sorry).

The lane load (Figure 3.7.6B) represents a train, or line, of trucks in a
lane, which consists of the "standard truck" (HS20-44, HS15-44, H20-44, or
H15-44 as shown in Figures 3.7.6A and 3.7.7A) preceded and followed by trucks
75 percent of the weight of the "standard truck."  See Appendix B.  (It used
to be that you had to consider the concentrated loads of each truck in the
train, or line, of trucks.  The lane loading made the arithmetic much
simpler.)

Section 3.11.4.2 should be read as:

"For continuous spans:

a. When lane loading (Figure 3.7.6B) is used, the loading shall be continuous
or discontinuous so as to produce the maximum effect (shear, moment,
deflection, etc.).

b.  When the standard truck loading (Figure 3.7.6A or 3.7.7A) is used (an H
or HS truck) only one standard truck shall be placed on the structure so as
to produce the maximum effect (shear, moment, deflection, etc.)"

The "standard truck loading" consists of a series of concentrated loads in
one or more lanes that represent the axle loads on one "standard truck" in
each loaded lane.

The "lane loading" consists of a uniform load in one or more lanes which may
be continuous or discontinuous, plus one or two concentrated loads in each
loaded lane, and represents the effect of a line of trucks traveling in those
lanes.

For a simple span, the standard truck loading generally controls for spans
less than approx. 140 ft. (See Appendix A) and the variable spacing, V for HS
trucks, is kept at the minimum distance, 14 ft.  For continuous spans when
the standard truck loading is used, the variable spacing, V, is adjusted,
primarily for determining negative moments, so that one heavily loaded axle
is on one side of the support and the other heavily loaded axle is on the
other side.

Hope this helps.

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

Ed Fasula wrote:

>>I just got the 16th Ed. AASHTO spec and I'm not clear what a lane load is
exactly.  3.7.1.1 states, "lane loads ... are equivalent to truck trains."

However 3.11.4.2 reads, "For continuous spans, the lane loading shall be
continuous or discontinuous; only one standard H or HS truck per lane shall
be considered on the structure."

Does "lane load" mean something different here?  If not, how can a lane load
represent a train of trucks yet be used when considering only that one is on
the structure?

Ed Fasula<<