Introduction - Design of Reinforced Concrete - page 29

1.20 INTRODUCTION TO LOADS

Perhaps the most important and most difficult task faced by the structural designer is the
accurate estimation of the loads that may be applied to a structure during its life. No loads
that may reasonably be expected to occur may be overlooked. After loads are estimated,
the next problem is to decide the worst possible combinations of these loads that might
occur at one time. For instance, would a highway bridge completely covered with ice and
snow be simultaneously subjected to fast moving lines of heavily loaded trailer trucks in
every lane and to a 90-mile lateral wind, or is some lesser combination of these loads
more reasonable?
The next few sections of this chapter provide a brief introduction to the types of loads
with which the structural designer must be familiar. The purpose of these sections is not
to discuss loads in great detail but rather to give the reader a “feel” for the subject. As will
be seen, loads are classed as being dead, live, or environmental.

1.21 DEAD LOADS

Dead loads are loads of constant magnitude that remain in one position. They include the
weight of the structure under consideration, as well as any fixtures that are permanently
attached to it. For a reinforced concrete building, some dead loads are the frames, walls,
floors, ceilings, stairways, roofs, and plumbing.
To design a structure, it is necessary for the weights or dead loads of the various parts
to be estimated for use in the analysis. The exact sizes and weights of the parts are not
known until the structural analysis is made and the members of the structure selected. The
weights, as determined from the actual design, must be compared with the estimated
weights. If large discrepancies are present, it will be necessary to repeat the analysis and
design using better estimated weights.
Reasonable estimates of structure weights may be obtained by referring to similar
structures or to various formulas and tables available in most civil engineering hand-
books. An experienced designer can estimate very closely the weights of most structures
and will spend little time repeating designs because of poor estimates.
The approximate weights of some common materials used for floors, walls, roofs,
and the like are given in Table 1.2.

Table 1.2 Weights of Some Common Building Materials

2 12 @ 16-in. double wood floor Reinforced concrete—12 in. 150 psf 7 psf

Acoustical ceiling tile 1 psf Linoleum or asphalt tile 1 psf

7
8

Suspended ceiling 2 psf Hardwood flooring ( -in.) 4 psf

Plaster on concrete 5 psf 1-in. cement on stone-concrete fill 32 psf

Asphalt shingles 2 psf Movable steel partitions 4 psf

1
2

3-ply ready roofing 1 psf Wood studs w/ -in. gypsum 8 psf

Mechanical duct allowance 4 psf Clay brick wythes—4 in. 39 psf