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-----Original Message-----
From: Peggy Dall [mailto:pdall(--nospam--at)metwood.com]
Sent: Monday, September 20, 2004 11:27 AM
To: seaint(--nospam--at)seaint.org
Subject:=20


Does anyone know where H20 loading comes from and what it defines?  I'm
= working on a small bridge and we want to be sure that we're designing
= for the correct loads.  Also, is there a special spec for say,
firetruck = loads?  Thanks in advance.=20 =20 P. Dall


Peggy,
The H- and HS- series trucks are defined in the AASHTO Bridge Design
Code.  They were first published as a standard design loading in 1944.

The H stands for Highway; the 20 stands for 20.  The H-20 truck weighs
20 tons: 16 on the rear axle and 4 on the front axle.  It is a straight
truck, not a semi-trailer truck.  It measures 14 feet between axles.  If
you pay much attention to trucks, you'll realize that a truck this short
will almost never weigh that much, and a truck weighing that much will
almost never be that short; moreover, the weight would be more evenly
distributed between the axles.  It's purely an imaginary design truck.
Its moment envelope will always exceed that of an equivalent weight real
truck, and its effect on the smaller parts of a bridge is exaggerated by
the overly heavy rear axle.  This is on purpose, as these are the parts
of a bridge that usually break (due more to their increased sesitivity
to dynamic loads and fatigue than actual load brought to bear).

An H-15 truck, as you've guessed by now, weighs 15 tons--3 on the front
and 12 on the back.  These are never used anymore, at least around here.

An HS-20 truck is similar, but has three axles; the S stands for
Semi-trailer.  The Tractor part is identical to an H-20.  The
semi-trailer is carried by another axle weighing 16 tons, so most
engineers think they should be called HS-36 rather than HS-20, because
they weigh 36 tons.  The guys that named it are mostly all dead now, so
I think we're stuck with it.  Anyhow, technically the distance from the
rear axle to the middle (driver) axle on an HS-20 can vary from 14 to 30
feet.  For simple spans the 14 foot measurement governs; for negative
moment on some continuous spans, the longer dimension comes into play.

When I've load-rated bridges for their ability to carry fire trucks,
I've always gotten the actual axle loads from the Fire Department,
because these can actually approach the axle loads of an H-series truck.
Acey is right in that the outrigger loads from a tall ladder truck can
far exceed the wheel loads, but this is almost never a consideration for
a bridge.  Conceivably it might govern the design of, say, an
underground parking ramp in front of a high-rise.  For the design of a
new bridge, however, we always use an HS truck.

I don't know who your client is, but I'd be very surprised if it's your
decision as to what the design load vehicle should be.  Personally, I
wouldn't design for less than HS-20.  Minnesota DOT's standard load for
some time has been HS-25, which weighs (paradoxically) 45 tons.

For the sake of completeness, I should mention that there are
alternative loads which are a part of an HS- specification.  One is a
uniform load with a large point load, referred to as the "lane load."
The other is called the "military loading," and is a pair of very heavy
axles 4 feet apart.  There is also a multiplier applicable to all these
variants to account for impact which varies depending on the span
length.

Finally, a few years back, AASHTO revised its design vehicle.  It is the
same old HS truck with a uniform load superimposed on it.  At the same
time, they revised the load factors for design, so a simple comparison
is difficult.  Some state DOT's have adopted this new vehicle and some
still use the old HS20-44.  Again, I'd be surprised if you will be the
one who gets to choose.

You need a copy of the AASHTO Bridge Design specs, if you don't have
one.

HTH,
Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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