When a particular type of structure is being considered, the engineers
may be puzzled by the question, "Should reinforced concrete or structural
steel be used?" There is much joking on this point, with the proponents of
reinforced concrete referring to steel as that material which rusts and those
favoring structural steel referring to concrete as that material which when
overstressed tends to return to its natural state-that is, sand and gravel.
There is no simple answer to this question, inasmuch as
both of these materials have many excellent characteristics that can be
utilized successfully for so many types of structures. In fact, they are often
used together in the same structures with wonderful results. The selection of
the structural material to be used for a particular building depends on the
height and span of the structure, the material market, and foundation
conditions, local building codes, and architectural considerations. For
buildings of less than 4 stories, reinforced concrete, structural steel, and
wall-bearing construction are competitive. From 4 to about 20 stories,
reinforced concrete and structural steel are economically competitive, with
steel having taken most of the jobs above 20 stories in the past. Today,
however, reinforced concrete is becoming increasingly competitive above 20
stories, and there are a number of reinforced concrete buildings of greater
height around the world. The 74 story 859-ft-high Water
Tower Place in Chicago
is the tallest reinforced concrete building in the world. The 1465-ft CN tower
(not a building) in Toronto ,
Canada
Although we would all like to be involved in the design
of tall prestigious reinforced concrete buildings, there are just not enough of
them to go around. As a result, nearly all of our work involves much smaller
structures. Perhaps 9 out of 10 buildings in the United States are 3 stories or less
in height, and more than two-thirds of them contain 15,000 sq ft or less of
floor space.
Foundation conditions can often affect the selection of
the material to be used for the structural frame. If foundation conditions are
poor, a lighter structural steel frame may be desirable. The building code in a
particular city may be favorable to one material over the other. For instance,
many cities have fire zones in which only fireproof structures can be erected-a
very favorable situation for reinforced concrete. Finally, the time element
favors structural steel frames, as they can be erected more quickly than
reinforced concrete ones. The time advantage, however, is not as great as it
might seem at first because if the structure is to have any type of fire
rating, the builder will have to cover the steel with some kind of fireproofing
material after it is erected.
To make decisions about using concrete or steel for a
bridge will involve several factors, such as span, foundation conditions,
loads, architectural considerations, and others. In general, concrete is an
excellent compression material and normally will be favored for short-span
bridges and for cases where rigidity is required (as, perhaps, for railway
bridges).
