MAKING STRONG BRIDGES - Bridges - page 52

those loads. Since about the year 2000, most engineers in the United States
have adopted a new method of bridge design. It is the method professional
engineering societies and many states now require—a method that recogniz-
es that, inevitably, uncertainties must be taken into consideration in design.
The method is called Load and Resistance Factor Design, or LRFD for
short. Preceding a fuller explanation, it is worth saying that the method (1)
adjusts load by uncertainty factors and (2) adjusts resistance by such factors;
hence the F in LRFD is the abbreviation for factor. (Engineers in Europe use
a similar method under a different name.) Participants in bridge decisions
should understand why this is the guiding method for bridge engineering:
why it serves to increase bridge safety.
To architects, design refers to the imaginative creation of the overall
structure, with a view to safety and cost for sure, but also to its fitness to
place and esthetic qualities as experienced by users. To bridge engineers,
design (sometimes known as engineering design ) has a more specific meaning.
It is the selection, arrangement, placement, and assembly of material and
components into a system that provides sufficient safety and serviceability
at acceptable cost. The challenge to the engineer is to know which system
provides just that.
Our purpose in the chapter is to explain the reasoning through which
the engineer accomplishes this design task. For those who will follow this
explanation, there is an important proviso. The vast majority of bridges are
of short-to-medium spans, so the engineer does not need to work through
the entire design logic explained here. Rather, he relies on professional
codes and standards, which specify the components, sizes, and materials
appropriate for normal circumstances. Even for these routine bridges, the
professional committees that drafted the codes had to base them on the
design principles (as well as research results and professional experience)
to be described here.
SETTING LIMIT STATES
The perceptive reader will say that this is now abundantly clear and hardly
worth saying: London Bridge may be falling down, but the bridge we design
certainly shouldn't. Our engineer should make sure that loads do not exceed
the structural limits at which damage occurs.
But our problem is not that simple. Everyone agrees that the bridge
must not collapse, but it may well become damaged enough to be unusable
or rickety long before collapsing. Or it may behave well enough for a while,
but undergo deformations and metal fatigue after two decades of stresses. So
the engineer has to decide the kind of damage he is trying to avert. The
upper threshold of resistance against which he sets projected loads depends
on the kind of damage being averted.