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QUERY: Fatigue

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I am doing a fatigue analysis of a steel cover for a shallow vault set in the
pavement of an interstate highway traffic lane. (Yes, it DOES sound like a dumb
place for it, but there are extenuating circumstances that are not germane to my
question, so just bear with me).

The basic design of the cover is a steel plate with angle steel attached via
fillet welds as "ribs" to the underside, with some rectangular steel bar
material running around the perimeter as a "frame", also fillet welded.

What I want is to run my approach by you, and get your comments. In particular,
I need some comment on the design for fatigue, which is not commonly done for
structures.

First of all, I have applied as much of the AASHTO Bridge Design Spec as I
possibly could, considering that this is a VERY special case. I applied a 75%
impact factor for example, which mirrors what is required for bridge expansion
joints, taking into account the commentary to the code that mentions this being
appropriate for "discontinuities" in the pavement. Since the box is small, of
course, I'm using a single "wheel footprint" of 10 inches by 20 inches, as
AASHTO describes. I'm using a 16,000 lb. wheel load per the AASHTO standard
truck.

This gave me a pretty hefty requirement for design for strength, and now I need
to look at this for fatigue.

Now, the design for fatigue under AASHTO uses a pretty good-size load of 75% of
the design truck load. However, I happen to know that this particular location
will see pretty much NO truck traffic. It will, however, see quite a lot of
transit bus traffic, and I have determined that a bus single-wheel load of
10,000 lbs. looks reasonable. I have decided to use this for fatigue, also
adding a 75% impact factor.

Since AASHTO is concentrated exclusively on trucks running on bridges, I've
decided that for fatigue, it doesn't give me the design procedure I want to use.
I have decided to use instead, the AISC procedure, which is more generalized,
but which also looks like it uses the same stress "threshold" as AASHTO (I
assume they are based on the same research).

I have a fairly accurate accounting of the number of bus trips per day, and a
reasonable estimate of bus trips per day. I am assuming that about one-third of
the bus wheels that run over this road will hit the cover, and all that together
tells me that I am looking at between 500,000 and 2,000,000 cycles over the
lifetime of the vault cover.

What I'm really wondering about, though, is how to apply the various "type and
location" requirements that are shown in Table A-K3.2 of AISC LRFD. I am
assuming that the critical elements are, of course, the "ribs", the bars making
up the "frame", and the attendant fillet welds. However, since these are
attached to the cover plate, it isn't clear to me how they need to be treated as
they don't seem to correlate with any of the "illustrative examples" found in
the AISC LRFD manual on pages 6-142 and -143.

Anyone have any suggestions?