A Beginner's Guide to ASCE 7-05

Chapter 3 - D: Dead Loads

2007, T. Bartlett Quimby

Overview

Typical Unit Area Dead Load Calcuations

Homework Problems

References


Report Errors or Make Suggestions

 

Section 3.1

Overview

Last Revised: 09/09/2008

ASCE 7-05 3.1.1 defines dead load as consisting "... of the weight of all materials of construction incorporated into the building...".  This is generally considered to mean that anything that is a fixed part of the structure is a dead load.  To be considered dead load, an item must be physically  attached to the structure.  

One test that generally works is that if it can be moved without cutting it loose or detaching it from the structure then it is not a dead load.

Items that can be considered to be dead load include construction materials that make up the building (beams, columns, floor systems, ceiling systems, wall systems, doors, windows, floor coverings, wall coverings, cabinets, and the like) and permanently attached equipment such as heating and ventilating systems, electrical trays, piping, etc.

Items that are not considered to be dead load include such things as movable shelving, desks, chairs, beds, chests, books, copiers, stored items, or anything else that can or may be moved around during the life of the structure.

One feature of dead loads is that they are the weights of the final structure.  This creates a bit of a dilemma for the design engineer.  The engineer must know the weights of the structure in order to design it, but the engineer also needs the final structure to accurately define the weights!  At the beginning of the design process, the framing and other structural elements are all unknown, however the weight is needed to determine the internal forces for the members being designed.  The solution to this dilemma often involves a few iterations where an educated guess is made as to where the design will end up, compute the dead loads based on this estimate, select members based on the estimated loads, recompute the dead loads, then continue the cycle until member sizes don't change. 

Expression of Dead Load in Structural Calculations

There are several convenient ways to express the intensity of dead loads. 

Floors, Roofs, and Walls

Most floor, roof, and wall systems have fairly uniform density and their weights can be expressed in terms of weight per unit area.  With dead loads expressed in this manner, tributary area concepts can be used to determine the forces exerted on the supporting members.

To compute the average unit weight (i.e. weight per unit area) such a system, the weights of all items in system are expressed in terms of their weight per square foot of surface area even if they are located as particular locations.  Generally, this method works if all items in the system they are either spread over the surface of the area (like carpet on a floor system... every unit area has it) or uniformly spaced over the entire area (like floor joists that support the floor deck and a ceiling below). 

Items that are not spread over the surface and not uniformly spaced over the entire area are generally not included in the unit weight calculation but are treated as individual loads in addition to the unit load. 

Miscellaneous Load

The exceptions to this are minor electrical and mechanical items such as wiring and plumbing whose exact location is unknown during design, as well as added density at the connections to supporting structures.  To account for these items an additional "miscellaneous" load may be applied.  The magnitude of the miscellaneous load generally varies depending on what you expect the extent of these items to be.  For typical cases, values between 1 and 2 psf are typical.

Slope Correction

Unit dead loads for roof areas that are sloped are commonly expressed in terms of weight per unit area of horizontal projection.  This is done because most load computations are done in terms of plan view unit areas.  The use of weight per unit horizontal projected area requires a slope adjustment as the weight of a sloped surface expressed in terms of weight per unit area of horizontal projection is larger than the weight per unit area along the surface. 

If the roof slope is 4:12 or less then the adjustment is less than 4% and not considered to be significant.  Consequently the slope adjustment is commonly done only for slopes greater than 4:12.  You will see this calculation in section 3.2.

Partition Loads

ASCE 7-05 section 4.2.2 classifies partition load as a live load, with no apparent explanation in the commentary.  The seismic requirements (ASCE 7-05 12.7.2) do explicitly require that the partition load be include in the effective seismic weight of the building.

Reroofing Allowance

Certain types of roofing systems allow for a new roof to be placed over the old when it is time to replace the roofing.  This is the case with asphalt shingles.  When determining the weights to be used for roofing, many engineers include a reroofing allowance for the added weight of the second layer of roofing material.  If in doubt, consult with the architect on the likelihood of the selected roof system requiring a reroofing allowance.

Sources of Dead Load Information

Given that area dead load calculations, there are a number of sources that present tables of unit weights expressed in terms of weight of per unit area.  The following are a couple that I have used in the past:

  • ASCE 7-05 Tables C3-1 and C3-2.  These tables, in the commentary, is quite extensive.  Many typical building materials are represented in the table.  Values in both USCU and SI units are presented.  Table C3-1 expresses values in terms of force per area for materials typically distributed uniformly.  Table C3-2 gives the densities of various materials so that weights can be computed. 
  • The AISC Steel Construction Manual, any edition.  Under the "MISC" tab you can find a table of weights of building materials plus weights and specific gravities for a wide variety of materials.  There is a table the gives weights of common building materials in terms of weight per unit area.  The table of weights and specific gravities generally express the weight in terms of weight per volume.  With some simple calculations, these values can be used to express weights in terms of weight per unit surface area.
  • Industry Catalogs.  Several manufactures (such as joist suppliers, truss manufacturers, etc) have included construction material typical weight tables in their literature to aid engineers in quickly computing the loads needed to specify their materials.
  • Structural Engineering Textbooks:  Many design texts include weight tables in their appendices.

You will find, when you compare sources, there is not complete agreement in the estimates that each source puts together.  This is to be expected.  As with using any tables, you need to apply a little engineering judgment, taking into account your actual job conditions.  When in doubt it is probably best to be mildly conservative with your estimate.  Being too conservative may have big ramification in structures with large dead loads.

Given the inexact nature of dead load computations, it is common to round dead loads to integer values of no more than three significant figures.

Linear Members

The dead load of linear members such as beam, columns and large pipes are generally expressed in terms of weight per unit length.

For large pipes that flow full, the contents can be considered to be dead loads because their weights and locations are very predictable.

Specific Items

Items that cannot reasonably be treated as being area or linear loads are treated on a case by case basis.  Equipment that is attached to the building is expressed in terms of the connection loads and are placed at the location of the connection.  For some calculations (generally for those members supporting but not directly attached to the item) items may be considered as point loads that act through their center of mass.