Because of their inherent

fl

exibility and low damping ratios, cantilevered mast

-

arm

tra

ffi

c signal structures are suscepti

b

le to

wind

-

induced vibrations.

These vibrations

cause stru

ctural stresses and strains to develop in a cyclical fashion which can

lead

to reduced service life due to fatigue and, in extreme cases, full collapse.

In 2001, after the collapse of several of these

structures throughout the United

States, American Assoc

iation of State Highway and Transportation O

ffi

cials (AASHTO)

code standards were updated to include fatigue provisions for tra

ffi

c signal supporting

structures.

In New York State, there is

particular concern regarding structures

spanning longer than 14 me

ters which currently do not meet these updated fatigue

provisions. To address this concern, a full

-

scale experiment was conducted using an

existing 25 meter mast

-

arm tra

ffi

c signal

structure, located in Malta, NY, in which

the response of the structure was

observed in relation to in

-

situ wind conditions.

In previous studies, high

-

amplitude vertical vibrations of mast

-

arm tra

ffi

c signal

structures have been shown to be due to

vortex shedding, a phenomenon in

which alternatingly shed, low

-

pressure vortices in

duce oscillating forces onto the

mast

-

arm causing a cross

-

wind response. When the frequency of vortices being shed

from the mast

-

arm corresponds to the

natural frequency of the structure, a resonant

condition is created. The resonant condition causes the l

ong

-

lasting, high

-

amplitude

vibrations

,

which may lead to the fatigue failure of these structures.

Turbulence in the approach

fl

ow is known to

a

ff

ect the cohesiveness of vortex

shedding. Results from this full

-

scale experiment indicate that the surrounding

terrain conditions, which a

ff

ect the turbulence intensity of the wind, greatly in

fl

uence

the likelihood of occurrence of long

-

lasting, high

-

amplitude vibrations and also

impact whether reduced service life due to fatigue is likely to be a concern