Civil Engineering Reference
In-Depth Information
Engineers refer to these damage thresholds as
limit states.
Through
guidelines developed by professional standards committees, bridge engineers
must (with some exceptions) make bridges conform to these defined limit
states. The American Association of State Highway and Transportation
Officials (AASHTO), the primary standards developer for bridge engineers,
identifies the limit states as
strength
,
service
,
fatigue
, and
extreme event
. As
the strength threshold is the one we will consider in this chapter, we sum-
marize the others just briefly.
The
service
limit refers to the quality of the bridge for its users. If in the
course of use the bridge is excessively bouncy, vibrates too much, causes too
much jarring, otherwise provides an unpleasant or frightening ride, reduces
traffic speed, and damage vehicles, then it is not properly providing its
intended service. The bridge may also crack at multiple points, tilt or dip
in a way that makes the driving surface uneven or causes traffic accidents,
or vary from rough to smooth to slanted to bouncy along different segments.
The bridge may exhibit these inferior forms of service quality while still
posing no danger of collapse. When designing a bridge, the engineer must
as one of his duties ensure that bridge performance does not fall below
standard limits of acceptable service quality.
The
fatigue and fracture
limit state refers to potential damage from
persistent cycles of traffic loads. During years of traffic, structural compo-
nents are subjected to repeated stresses, which may initiate and then over
time exacerbate cracks, deformations, fractures, and damage to connectors.
The cumulative damage reduces service quality and can even result in cata-
strophic fractures, which undermine bridge strength, leading to collapse. As
fatigue occurs from stresses applied repeatedly over long durations, it requires
special methods of analysis.
The
extreme event
limit state seeks the structure's survival during earth-
quake, severe collisions, ice flows, and scour at the bridge foundations. We
return to this subject in the next chapter.
Now we get to the threshold of acceptability on which we concen-
trate for the rest of the chapter. It is the one most people would expect
to be the determining factor in bridge engineering: the
strength
limit state.
For a structural member, say a girder, strength is the stress it can undergo
without fracture or other serious damage threatening collapse: the greater
the strength, the higher the stress the structural member can withstand.
The strength limit of a cable in tension is determined in part by the elastic
limit, beyond which the cable stretches precipitously for added units of load.
For a bridge assembly as a whole, the strength limit state is the stan-
dard for the highest combination of loads the structure can withstand while
retaining its structural integrity. The strength limit state is set at the thresh-
old beyond which the structure undergoes distress, structural integrity is
impaired, and repair is required (but collapse is not imminent).
