Page 232 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 207

A (Typical)
Web (Typical)
Bottom Flange (Typical)

A Elevation (Typical)
Fillet Weld (Typical) (A)
Cover Plate at Bottom (Typical)
Flange Width Cover Pl. Width

(B) Plan (Square Ends Welded)

Flange Width Cover Pl. Width

(C) Plan (Square End Not Welded)
Bottom Flange
Flange Width Cover Pl. Width

(D)
Wide Cover Plate Plan (Rounded Corners)

Flange Width Cover Pl. Width

Sectional Plan (Over Sized Not End Welded
Looking Down on Bottom Flange)
Figure 5.8 Possible crack propagation at cover plate ends.

• Internal or member redundancy: A configuration that contains three or more elements that

are fastened mechanically such that multiple independent load paths are formed. Failure
of one member element would be local and would not cause multiple member failure.
• Intersecting welds or welds which overlap or are closely located will have high stress con-
centration. Examples are welds located at the intersection of a transverse and longitudinal
stiffener. Tack welds or weld repairs may also reduce the internal or member redundancy.
Transverse welds that are perpendicular to the applied stress field exhibit a greater tendency

to crack than continuous longitudinal welds that are parallel to the applied stress fi eld.
The average volume of traffic per day shown on any route would vary according to rush

hours, weather conditions, or for an important event. Some lanes may be closed occasionally
due to accident or repair. Some states, such as Michigan, have the highest truck loads in the
country. There is little control over intensity of out-of-state traffic using interstate highways.

Also, projected traffic in 20 years may be much higher.
Traffic counts are needed for the planning of each bridge and for fatigue studies. It would

be uneconomical to design bridges located on local roads for the same intensity of truck load as
the heaviest interstate trucks. Also, the fatigue of girders will be proportional to the frequency
and intensity of trucks.
Individual traffic counts may only be representative of the time when they were taken. Even

an envelope of counts made over a long period may change due to detours implemented elsewhere.
At best, these are approximate and are guidelines only; however, as a check a system of weigh
stations has been installed that permits loads specified by different states.