Introduction - Design of Reinforced Concrete - page 30

1.22 LIVE LOADS

Live loads are loads that can change in magnitude and position. They include occupancy
loads, warehouse materials, construction loads, overhead service cranes, equipment oper-
ating loads, and many others. In general, they are induced by gravity.
Some typical floor live loads that act on building structures are presented in Table
1.3. These loads, which are taken from Table 4-1 in ASCE 7-02, 19 act downward and are
distributed uniformly over an entire floor. By contrast, roof live loads are 20 psf maxi-
mum distributed uniformly over the entire roof.
Among the many other types of live loads are:

Traffic loads for bridges. Bridges are subjected to series of concentrated loads of
varying magnitude caused by groups of truck or train wheels.
Impact loads. Impact loads are caused by the vibration of moving or movable loads.
It is obvious that a crate dropped on the floor of a warehouse or a truck bouncing
on uneven pavement of a bridge causes greater forces than would occur if the loads
were applied gently and gradually. Impact loads are equal to the difference be-
tween the magnitude of the loads actually caused and the magnitude of the loads
had they been dead loads.
Longitudinal loads. Longitudinal loads also need to be considered in designing
some structures. Stopping a train on a railroad bridge or a truck on a highway
bridge causes longitudinal forces to be applied. It is not difficult to imagine the
tremendous longitudinal force developed when the driver of a 40-ton trailer truck
traveling at 60 mph suddenly has to apply the brakes while crossing a highway
bridge. There are other longitudinal load situations, such as ships running into
docks and the movement of traveling cranes that are supported by building
frames.
Miscellaneous loads. Among the other types of live loads with which the structural
designer will have to contend are soil pressures (such as the exertion of lateral
earth pressures on walls or upward pressures on foundations), hydrostatic pres-
sures (as water pressure on dams, inertia forces of large bodies of water during
earthquakes, and uplift pressures on tanks and basement structures), blast loads

Table 1.3 Some Typical Uniformly Distributed Live Loads

Lobbies of assembly areas 100 psf Classrooms in schools 40 psf

Dance hall and ballrooms 100 psf Upper-floor corridors in schools 80 psf

Library reading rooms 60 psf Stairs and exitways 100 psf

Library stack rooms 150 psf Heavy storage warehouse 250 psf

Light manufacturing 125 psf Retail stores—first floor 100 psf

Offices in office buildings 50 psf Retail stores—upper floors 75 psf

Residential dwelling areas 40 psf Walkways and elevated platforms 60 psf

19 American Society of Civil Engineers, 2002, Minimum Design Loads for Buildings and Other Structures ,
ASCE 7-02 (Reston, VA: American Society of Civil Engineers), pp. 12-15.