Basic Design Concepts
© 2006,2008 T. Bartlett Quimby
Loads and Their Combinations
Last Revised: 08/02/2008
It is your responsibility as the structural engineer to design safe, serviceable structures. In order to do so, you must predict the magnitudes of the various loads that are likely to be applied to the structure over it's life time. You must also account for the probability of the simultaneous application of the various load types.
In order to bring consistency to the prediction of loads, the profession has adopted standards that dictate the loads and their probable combinations that must be used in design. Be aware that these loads and combinations is not necessarily comprehensive. There may come a time when, in your professional judgment and that of your peers, that there will be the need to exceed the values set in standards.
The current standard for determining loads on buildings is ASCE 7, Minimum Design Loads for Buildings and Other Structures. The 13th Edition of the SCM is based on the 2002 version of ASCE 7 (aka ASCE 7-02). A new version, with some changes to the load combination equations, has been released as ASCE 7-05. The ASCE 7 document is a "must have" document for all structural engineers practicing in the United States. All building related design specifications in the United States reference this document. You will need to see ASCE 7 for specific details on the various types of loads and their detailed application. Fortunately, for this text, you will generally be given loads and told what type they are so you do not need to know the details.
For this text we use the latest version of ASCE 7.
At this point, if you have ASCE 7, you can look up the discussion regarding the load types. Alternatively, if you have the SCM turn to the section on Loads, Load Factors, and Load Combinations (SCM pg 2-8) to follow along with the following conversation.
The principle load types from ASCE 7 are listed in the referenced section. The following is a brief definition for reach load type along with a short discussion about the natures of the loads. Understanding the natures of the loads will help you to understand the choice of load factors. You are strongly encouraged to study the definitions more fully in ASCE 7 and other relevant references.
Dead Load, D
The dead load on a structure includes the weight of all items that are attached to the structure and are likely to remain in the as-built location throughout the life of the structure. Beams, columns, floor slabs, exterior walls, roofs, mechanical equipment, and the like are all considered to be dead load on a structure. Inanimate objects that are not physically connected to the structure and/or may be moved around during the life of the structure are not considered to be dead loads. For example: tables, chairs, desks, file cabinets, shelves, and the like are not normally included in dead load estimates. Dead loads can be computed accurately with a relatively high degree of confidence.
Live Load, L
Live load includes anything that can possibly be moved in or out of the structure over the course of its life. This includes people, furniture, equipment, and other similar items. Predicting the live load that a structure will see is highly dependent on the use, or that the structure will be put to.
The type of use is normally referred to as the "occupancy" of the structure. As the occupancy of the structure may change over its life, reasonable assumptions about its future must be made. If the occupancy of a structure changes to one expected to see heavier loads then modifications may need to be made to accommodate the increased load.
Different parts of structure will be assigned different live loads depending on their use. For example, exit corridors (hallways and stairs) need to be designed for higher loads than an office space.
Live loads tend to be transient so durations of sustained live load are somewhat less than the life of the structure. The length of duration will vary with occupancy.
Accurately predicting the live load that a structure may see over its life time is very difficult. Values listed in ASCE 7 are based on experience, measurements, and probability. There is a chance that the values listed may be exceeded at some time, so caution is justified in accounting for these loads.
Another very important characteristic (one that will have a great effect on analysis) of live loads is that live load need not be everywhere present at a given time. The design codes required that it be placed for maximum effect. In continuous structures this generally means that you will need to solve for multiple load cases in order the find the envelope of required strength values needed in order to design a safe structure.
Roof Live Load, Lr
Roof live load is generally associated with the loads that the roof structure will see during construction and later during maintenance (i.e. during reroofing). These loads are of short duration and generally much smaller than normal live loads since it is not expected that roofs will see the types of loads that floors see.
Snow Load, S
Snow loads occur in colder climates and are of varying duration. Snow, unlike live load, is considered to everywhere present at a given time. The magnitude of snow load is highly dependent on local weather patterns, terrain, and latitude. Snow drifting must also be considered when snow loads are present.
The nature of snow load it is as predictable as mother nature! Where there are extensive records, the design snow load can be statistically determined, however, it is not uncommon to have unusual snow events in cold regions that may exceed the design values.
In cold regions, snow load values may be in excess of roof live loads, making roof live loads irrelevant as a design consideration.
Rain and Ice, R
Rain and/or ice loads are similar to snow loads in their predictability. As noted in ASCE 7 and the SCM, R is exclusive of ponding. Ponding loads are more predictable and are treated separately.
Wind Load, W
Wind load is a very dynamic event for which static approximations can be made. The approximate methods for determining wind load ASCE 7 are generally considered to be conservative for a given predicted wind speed, however wind speed is a difficult thing to predict. The probability of exceedance is relatively high.
Earthquake (or Seismic) Load, E
Earthquake forces are generated by very dynamic events. For certain types of structures a static equivalent method may be used to estimate the forces applied to the structure. For more complex structures numerical methods that solve the dynamic problem must be used.
Earthquake loads are unique in that they are the only load that we compute at ultimate strength levels. All others are computed as service (or actual) strength levels. The actual forces generated in structures by earthquakes are so large that it is not normally financially feasible to design building structures to elastically withstand them. As a result, there are detailed requirements to ensure structures are ductile enough that they are not likely to collapse during an earthquake, thus allowing the occupants to escape. Since ductile behavior is expected, the loads computed are computed at the strength level of the structure.
Combining the Loads
Many structures will see most, if not all, the loads listed above sometime in their life. The next challenge becomes how to combine the loads reasonably. A direct combination of all the loads at their maximum is not considered to be probable. For example, it would not be reasonable to expect a full live load to occur simultaneously with a full snow load during a design level wind storm.
ASCE 7 provides load combination equations for both LRFD and ASD loads. Your choice will be based on the design philosophy that you are using.
When using the 13th edition of the SCM, notice that it lists (pg 2-8) a subset of the load combinations found in ASCE 7-02. As is the nature of the industry, ASCE 7 has subsequently been updated and released as ASCE 7-05. There are some changes to the load combination equations in the latest ASCE 7. We will be using these.
You need to visit A Beginner's Guide to ASCE 7-05, Chapter 2 for the full discussion on load combinations. We will be using the load combination definitions presented there. There is also an example problem in the BGASCE7 chapter that illustrates the application of the load combination equations.
Load and Resistance Factor Design
If you chose to use LRFD for your design philosophy, then you are to make sure that your structure is capable of supporting the seven ASCE 7-05 basic load combination equations.
Loads computed using the LRFD load combinations will have the subscript "u" in these notes and in the SCM. For example: Pu, Mu, Vu, and Ru.
Allowable Strength Design
For ASD there are seven basic load combination equations.
Loads computed using the ASD load combinations will have the subscript "a" in these notes and in the SCM. For example: Pa, Ma, Va, and Ra.
Comparing ASD vs. LRFD
A Beginner's Guide to ASCE 7-05, section 2.4 has an example that illustrates the variability of the LRFD factor of safety in relation to the ASD fixed factor of safety.