Bridge Series: Tied-Arch Bridge Traditional Cache
Bridge Series: Tied-Arch Bridge
-
Difficulty:
-
-
Terrain:
-
Size: 
(micro)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
Tied-Arch Bridge at NE 124th Street Spanning The
Snoqualmie River
A tied-arch bridge is a modified truss bridge in which the outward
horizontal forces of the top chord (arch) are resisted by tension
in a bottom chord rather than by the bridge foundations
(abutments). Or put another way, the ends of the arch are tied
together in the bridge structure so that the forces balance, much
like a bow. For this reason as well as the obvious visual
similarities, a tied-arch bridge is often referred to a bow-string
truss.
Tied-Arch Bridge Diagram
A basic arched bridge requires large abutments to resist the
outward thrust of the arch. Therefore they are usually located
where the soil grade is steep on both ends of the bridge, such as a
valley with cliffs on each side. A good example is the Glen Canyon
Bridge in Arizona.
Glen Canyon Bridge in Arizona
Where this condition does not exist, such as at NE 124th Street
over the Snoqualmie River, the bridge design is modified to include
tension ties between the arches.
There are several reason to choose a tied-arch design. Since much
of the supporting structure is above the bridge deck, they can be
installed closer to grade while still providing the necessary
clearance underneath. In this case it results in less ramp-up of
the road bed approaches and therefore the road remains relatively
level. This saves construction costs and the result is more
aesthetically pleasing too.
This bridge design is what I like to refer to as the Washington
State Default Bridge. I’m not sure why but WSDOT seems to like
these very much and they are located all over the state. They are
very common on I-5 both north and south of Seattle.
The Cache
The cache is small so bring a pen. Most trackables probably won’t
fit.
I believe this cache to be wheelchair accessible but upped the
terrain rating to 1.5 due to traffic.
There is a spot for pull-off parking nearby and the road has a very
wide and paved shoulder running from there all the way across the
bridge. Heck, it’s even wide enough for a car and one might be able
to park right next to GZ.
Additional Hints
(Decrypt)
Zntargvp