Asce 7-05 Load Combinations

[Raymond Freedman]

You will note that several of the load combination equations have multiple permeations due to use of "or" or "+" in the equations (both wind, W, and seismic, E, are considered to be + loads). This is true of both the LRFD and ASD combinations.

If you chose to use LRFD for your design philosophy, then you are to make sure that your structure is capable of supporting the loads resulting from the seven ASCE 7-05 basic load combination equations.

LRFD applies load factors to service level loads so that they are safely comparable to member strengths (which are generally inelastic) while maintaining the actual (service) loads in the elastic region. Member strength (the maximum load that the member will support) is generally between 1.3 to 1.4 times the force that will cause yielding in a member. These load factors are applied in the load combination equations and vary in magnitude according to the load type.

The magnitude of the LRFD load factors reflect the predictability of the loads. For example, the load factor for D is generally lower than the load factor for L in any given equation where there is equal probability of simultaneous occurrence of the full value of each load type. This is because dead loads are much more predictable than live loads and, hence, do not require as great of a factor of safety.

Example: Analysis of a structure shows that a particular member supports 5 kips dead load and 6 kips live load. Using LRFD LC-2, the combined design load equals 1.2 times the dead load plus 1.6 times the live load, or 15.6 kips. The factor for dead load (1.2) is lower than the factor for live load (1.6) because dead load is more predictable than live load. The load factors are all greater than 1.0 since we want to compare the result to the ultimate strength of the member instead of the yielding strength of the member yet we don't want yielding to occur. The ultimate strength is generally about 1.3-1.4 times the yield strength of the member.

For ASD there are eight basic load combination equations. You will notice that the large load factors found in the LRFD load combinations are absent from the ASD version of the ASCE 7-05 load combination equations. Also, the predictability of the loads is not considered. For example both D and L have the same load factor in equations where they are both likely to occur at full value simultaneously. The probability associated with accurate load determination is not considered at all in the ASD method. Hence the major difference between LRFD and ASD.

Example: Analysis of a structure shows that a particular member supports 5 kips dead load and 6 kips live load. Using ASD LC-2, the combined design load equals the dead load plus the live load, or 11.0 kips. The factor for dead load (1.0) is the same as the factor for live load (1.0), hence not accounting for the fact that the dead load is more predictable than the live load. The result of the load combination equation is then generally compared against the yielding strength of the member to ensure elastic behavior.

The following table shows the load combinations that have been considered for our proposal. These are in accordance with ASCE/SEI 7-05. If other load combinations are required, please specify them and request a revision to this proposal.

I attached five pictures, the first three pictures shown the load combinations explanation in ASCE code (Live roof and wind not included). the fourth and fifth for robot automatic load combinations and SDS value which is required for load combinations as per ASCE code. the program generate load combinations with some different (Like combinations 7,8,9 etc) also the vertical seismic load (0.2*SDS*D) its not considered in the robot combinations. any one can help regarding that. May be I have some missing definitions in programs.

A load combination results when more than one load type acts on the structure. Building codes usually specify a variety of load combinations together with load factors (weightings) for each load type in order to ensure the safety of the structure under different maximum expected loading scenarios. For example, in designing a staircase, a dead load factor may be 1.2 times the weight of the structure, and a live load factor may be 1.6 times the maximum expected live load. These two "factored loads" are combined (added) to determine the "required strength" of the staircase.

Section 12.4.3.3 of ASCE 7-05 (or -10) deals with overstrength (Ωo) load combinations and allows a 1.2 increase in allowable stress when using these combinations. We received a question from a customer last week asking if the 20% increase applies to Simpson Strong-Tie connectors. The simple answer is yes. When demand loads are based on amplified seismic forces, connector allowable loads may be increased by 1.2 per Section 12.4.3.3.

Since the increase may be combined with the duration of load increases permitted in the NDS, you would apply the 1.2 increase to connector allowable loads at a load duration of 1.6, which makes the overstrength factor a little less terrible.

ASCE 7 and other model building codes acknowledge that structures will be loaded beyond their elastic range during seismic events. Damping and ductile yielding make it unnecessary to design for the full inelastic design force, so the code divides the seismic response by the R-factor to get a lower elastic design force or base shear. Higher R-factors represent more ductile systems and, therefore, yield a lower seismic design force. Deflections are multiplied by the Deflection Amplification Factor, Cd, to obtain the expected inelastic deflections. Similarly, the System Overstrength Factor, Ωo, is an amplification factor that is applied to the elastic design forces to estimate the maximum expected force that will develop.

ASCE 7 Section 12.3.3 addresses limitations and additional design requirements for structural systems with irregularities. Tables 12.3-1 and 12.3-2 define horizontal and vertical structural irregularities and reference the code requirements applicable to each type. In some cases, the irregularities are simply prohibited for high seismic areas.

Many of the irregularities are allowed, albeit with additional design requirements that make use of the load combinations with the overstrength factor. The purpose of applying the overstrength load combinations to irregularities is to prevent non-ductile failures in the structural system.

Designing for amplified forces can be a real challenge, but the alternative would be the building code not allowing structural irregularities at all, which would not be realistic. I have always thought of the overstrength factor, Ωo, as being a sensible compromise.

The code has become to complex. Most buildings in USA can have a simple design load, and the penalty would be 5% for the structure, and less than .005% for the cost of the building. What is wrong wiih haveing extra reserve strength in buildings. The final cost is not much. The way real building are designed is with simple loads simple concepts. Who knows how a computer designs a building. We ignor parts of the Code We do not know what the loads are. WE know how to design so it does not fall. That is great . Spend the extra 0.005%

There is a change to the provisions of ASCE 7-05 noted in the blog post on the overstrength factor that are now found in ASCE 7-10. In Sec. 12.4.3.3, the NDS load duration factor is expanded to all NDS adjustment factors. Also, there are also requirements In Sec. 12.2.5.2 and 12.10.2.1 of ASCE 7-05 and 7-10, requiring use of the overstrength factor.

I have also had trouble determining when to apply O.S. in my residential light framed wood structures, or any structure for that matter. Are you saying, as C Jones summarized, that they are only applicable when we have structural irregularities, in order to compensate for them? That seems too simplified. They are clearly required in Sec. 12.2.5.2 and 12.10.2.1.

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Late last year, the Structural Engineers Association of California (SEAOC) learned that some engineers designing ballasted rooftop solar arrays using Allowable Stress Design (ASD) have not been applying the appropriate load combination for wind uplift (i.e., Load Combination No. 7a from ASCE 7 Section 2.4.1), either ignoring it or modifying it based on an incorrect understanding of its basis. In response, the organization issued the statement shown in the inset on the next page.

The authors have recently found cases in which this erroneous design practice is still taking place; thus, we are working to get the word out to engineers and agencies that work with solar energy structures and building officials. Please help by forwarding this article or the link to the SEAOC statement to anyone you know who might be interested.

The Structural Engineers Association of California (SEAOC) Wind Committee has recently learned that some engineers designing ballasted rooftop solar arrays for wind uplift have been using the Allowable Stress Design (ASD) load combinations of the ASCE 7 standard, the International Building Code (IBC), and the California Building Code (CBC) incorrectly.

The justification that SEAOC has seen for this practice assumes that the 0.6 factor on dead load is intended only to represent uncertainty in the dead load. This assumption is incorrect. Rather, the factor was derived so that ASD load combinations would give results similar to designs that use Load and Resistance Factor Design (LRFD). The degree of certainty that one may have in the dead load is not a justification to change this factor.

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