In article ,
Adam H. Kerman wrote:
Robert Bonomi wrote:
Stephen Sprunk wrote:

Load limits are specified per axle or tandem, and tank transporters have
_lots_ of axles to spread the tank's weight out.

Some are, some are *not*.

'More axles' doesn't make any difference to a bridge span. grin.

Can you expound on that? Bridges have a rated weight limit. If truck
weight is at the limit, why wouldn't the bridge benefit from improved
live load weight distribution, particularly if truck length exceeds
span length?

An extreme case -- if the static weight of the vehicle exceeds the load at
which the span will collapse, and the span is longer than the wheelbase, it
doesn't matter whether it's a unicycle, or has wheels every 2 ft.

Or, consider a short span, just under the vehicle wheelbase. You can move
a two-axle load over that span that is nearly twice the 'collapse' loading,
because only half the load will be on the span at any time. Add a 3rd
axle, at the midpoint, and the total load on the span goes -up-.


Also, bridge spans, in general, tend to have a 'crown' along the length of
the span, as well as the side-to-side crowning. A side effect of that
longitudinal crowning is that interior axles carry somewhat more weight
than leading/trailing ones.

Robert Bonomi wrote:
Adam H. Kerman wrote:
Robert Bonomi wrote:
Stephen Sprunk wrote:

Load limits are specified per axle or tandem, and tank transporters have
_lots_ of axles to spread the tank's weight out.

Some are, some are *not*.

'More axles' doesn't make any difference to a bridge span. grin.

Can you expound on that? Bridges have a rated weight limit. If truck
weight is at the limit, why wouldn't the bridge benefit from improved
live load weight distribution, particularly if truck length exceeds
span length?

An extreme case -- if the static weight of the vehicle exceeds the load at
which the span will collapse, and the span is longer than the wheelbase, it
doesn't matter whether it's a unicycle, or has wheels every 2 ft.

Yes, I see your point on that.

Or, consider a short span, just under the vehicle wheelbase. You can move
a two-axle load over that span that is nearly twice the 'collapse' loading,
because only half the load will be on the span at any time. Add a 3rd
axle, at the midpoint, and the total load on the span goes -up-.

Yes, I see your point on that as well.

Also, bridge spans, in general, tend to have a 'crown' along the length of
the span, as well as the side-to-side crowning. A side effect of that
longitudinal crowning is that interior axles carry somewhat more weight
than leading/trailing ones.

Ok.

But what about the way spans are designed to flex? There are several
trigonometric formulas that apply (that I never learned). Aren't there
instances in which the same live load on various wheelbases can positively
or negatively impact the span's flexibility by creating different kinds
of deflection?

In article ,
Adam H. Kerman wrote:

But what about the way spans are designed to flex? There are several
trigonometric formulas that apply (that I never learned). Aren't there
instances in which the same live load on various wheelbases can positively
or negatively impact the span's flexibility by creating different kinds
of deflection?

Things get complicated, and messy, in the real world. wry grin

Longer distances between axles can serve to spread the load over a larger
part of the spam, without increasing peak loading.

Additional axles, on the same overall wheelbase, can reduce the 'rate of
change' of the load at a particular point, BUT they can also _increase_ the
peak load at a particular point. This can be significant, at/near the weakest
point -- mid-span.

'Drive' wheels apply different forces on a span than "non-drive" wheels do.
Powered axles, in addition to the 'down' force of the load, provide a 'push'
towards the rear of the vehicle. Un-powered ones have the effect of adding
a push towards the -front- of the vehicle. This adds increased compression
effects _between_ the drive and non-drive axles, an a reduction before the
drive axles, as well as after the non-drive ones. These forces aren't large,
but can make for some significant changes as they pass over the mid-span
'divide'.