A Beginner's Guide to ASCE 7-05

Chapter 5 - L: Live Loads

© 2007, T. Bartlett Quimby

Section 5.2

Uniformly Distributed and Concentrated Loads

Last Revised: 11/04/2014

ASCE 7-05 Table 4-1 tabulates the uniformly distributed and concentrated loads for various occupancies.  There are quite a few footnotes to this table!  Make sure that you refer to them as needed.

Note that ASCE 7-05 section 4.3 states that the uniform load OR the concentrated load is used "whichever produces the greater load effects."  This means that you do not apply the two of them together!  This section also states that, unless otherwise specified, that the concetrated load is assumed to be distributed over an area that is 2'-6" square that is to be placed so as to cause maximum effect.  Figure 5.2.1 illustrates distribution of a 2,000 lb concentrated load on an area 2'-6" square.  Note that the location of this concentrated load moves depending on what you are trying to accomplish.

Figure 5.2.1
Distribution of Concentrated Load

The concentrated loads may be distributed over a small area.

For example, consider a floor joist that has a constant tributary width of 4 ft in an area that has office occupancy.  The uniform distributed live load and the concentrated load from ASCE 7-05 Table 4-1 are 50 psf and 2000 lbs, respectively.

Figure 5.2.2 illustrates the live load cases that would be considered for the joist.

Figure 5.2.2
Live Load Cases

Note that the entire concentrated load ends up on this joist since the joist spacing is larger than the 2.5 ft square area stipulated for the application area.  Most of the time, designers find it more convenient to apply the force as a point load at the center of the area, which can be done with adversely affecting the computations.  Both versions are shown in the figure.

Also note that the concentrated load is used twice.  It is placed at mid-span so as to create the maximum moment in the joist.  It is then placed near one support so as to create maximum shear in the joist.  The joist would be designed for the envelopes of all the moments and shears generated by these three load cases added to the dead load case.

Partition Loads

For some occupancies, particular office occupancy, the location of interior partitions is likely to change over the course of a building's life span.  These are fixed partitions or walls attached to the floor and/or roof system.  If it appears probable that the interior non-bearing walls and partitions will be rearranged over the life of the structure it is common practice to add a partition loading to the floor dead load to account for these walls.  Typical values range from 15 psf to 25 psf depending on the engineer's judgment of what is likely to happen.  The architect should be consulted on this subject.

As stated in ASCE 7-05 C4.2.2, the addition of partition loads to ASCE 7 is new with this edition of the specification.  Other building codes have had the requirement for some time.  The commentary does mention that partition loads have sometimes been considered to be dead loads but offers no explanation as to why they are considered to be live loads in this edition of the specification.

The seismic requirement (ASCE 7-05 12.7.2) does explicitly require that the partition load be include in the effective seismic weight of the building.

Other Distributed Live Loads

The commentary section, ASCE 7-05 C4.2.1 has an excellent discussion on how the values were obtained as well as some suggested values for other commonly encountered situations that are not part of the main table 4-1.  This information is to be used when negotiating live loads for various conditions with the local building authority.

Another item that is particularly useful for understanding the nature of some live loads and the mathematics behind the tables is ASCE 7-05 Table C4-2.  In this table you are given the results of load surveys and the interval and recurrence of the peak transient loads.  It is worth spending some time reviewing this table.