This section discusses periodic and permanent monitoring of bridge decks to assess their stability. 

The stability of bridge decks has been of concern to designers and owners ever since the infamous destruction of the Tacoma Narrows Bridge in 1940 (figure 64) under moderate winds.  Geophysical instruments, including accelerometers and tilt meters, as well as strain gages, are increasingly used for short- and long term monitoring of bridge decks.  Issues regarding stability monitoring and methods used will be discussed here.  There is additional information on vibration monitoring provided in this web manual under Vibration Measurements.

Figure 1.  Collapse of Tacoma Narrows Bridge. (Ed Elliott, The Camera Shop, Tacoma, WA)

The basic problems considered here are summarized briefly as follows:

1. The aerodynamic stability of bridge decks when subject to high winds. 
2. The long-term stability of bridge decks due to fatigue caused by a variety of cyclic stresses including temperature, increased loading from traffic, and heavily loaded trucks. (See http://www.pubs.asce.org/ceonline/1098feat.html and http://www.kinemetrics.com/newprojects.html#newprojects.bangkok)
3. The stability of bridge decks during earthquakes. (See http://www.kinemetrics.com/halkis.html)

The primary method used to evaluate stability is to measure the baseline performance characteristics of a structure and then monitor the change in these characteristics with time.  This can be done periodically, such as during an inspection cycle, or continuously using a permanent monitoring system.

The geophysical techniques applied to this problem are principally vibration monitoring, including strains, displacements, rotations, and accelerations, but also some environmental monitoring including wind and temperature, since these affect the response.