SLRS Basic Explaination 2

[WiSEiwish]

Seismic rookie here.

Can someone give an elementary explanation of how the seismic load resisting system is integrated with a building? From my understanding, the SLRS is meant to prevent the collapse of a structure in the event of a large earthquake, but that damage to the structure is expected. Does this mean that there are essentially two different lateral support systems in a single building? If this is the case then is the SLRS considered as part of the main lateral support while the other braces/moment frames that are not detailed for seismic not included in the seismic load combinations?

Any and all information and/or good reads would be helpful.

Thanks.

 

[jdgengineer]

I'll give you a very brief description. I'll probably omit important information, but hopefully point you in the right direction. I'll let someone else chime in on good reading material. I'm sure there are a lot of good resources in FEMA or NEHRP documents amongst other sources. FEMA 450 is a good reference that comes to mind. It is the basis for ASCE 7.

If you were to design a building that is rigidly attached to the ground (i.e. not base isolated) to resist the full maximum considered earthquake forces in a high seismic area and have the building remain elastic (i.e. not yield) you would have a seismic force resisting system that would be incredibly architecturally limiting and would be most likely cost prohibitive. Therefore, it is standard practice in the US to design for forces that are considerably lower than the elastic seismic forces (i.e. R factor). Designing for these lower seismic forces comes with a price and that is the fact that you are assuming the building will yield and will therefore have considerable nonlinear behavior and associated damage. In order to accommodate these large deflections and provide the required ductility to reach these deformations the building code has prescribed minimum detailing requirements for the lateral system. Typically, the yielding of the lateral system is isolated to very specific, stable, locations of the lateral system and the rest of the lateral system is designed based on a Capacity Based Approach. For instance, for a moment frame the yielding is restricted to occur in the beam (with an RBS it occurs at the center of the dogbone). The rest of the frame (columns, connections, etc.) are designed to the expected capacity of the beam so that the beam will remain the "fuse".

Typically, there is only one lateral system in a building. This lateral system is designed for the worst case forces between wind and seismic. There can be times where wind forces are higher (if you are in a moderate seismic area), but the seismic detailing requirements still govern (i.e. the wind forces were only higher because you got to divide the seismic forces by R). Any lateral system that is not detailed to the requirements for the R of your system is neglected because it will not have the required ductility to accept the deformations that are needed to use the lower seismic forces that we designed for.

Hypothetically, in a high seismic region you could avoid any special seismic detailing and design your system with a lower R. However, the building code does not typically allow this as they want you to provide a minimum level of ductility for your system (even if your calculations prove that it would not be required) so that in the event a seismic event is larger than what is designed for your building will still be able to have a stable post-yield mechanism.

While not directly stated in the building code it is my understanding that the building codes design intent is to provide approximately a collapse prevention limit state at the maximum considered earthquake (often considered to be a 2% in 50 year event - MCE event, ~2500 year return period depending on your seismic area). It's my understanding, that it is felt that the building will have an inherent margin aagainst collapse of approximately 1.5 when designed according to the building code so that rather than check the full 2% in 50 year event we design to 2/3 of these forces (SDS = 2/3 SMS). In past building codes we designed to a life-safety at a Design Basis Earthquake (often considered to be a 10% in 50 year event - DBE event, ~ 500 year return period depending on your seismic area). To make things confusing in some parts of the country you can approximately convert from the MCE event to the DBE event by taking 2/3 of the MCE forces. However, its my understanding that this is not the intent with SDS = 2/3 SMS, but instead is to account for inherent overstrength as mentioned previously. Theres also deterministic and probabilistic ground motions which define the design forces throughout the country but I won't get into that here.

There are typically 4 performance levels that are considered (in today's codes at least) with seismic performance. These include: Operational, Immediate Occupancy, Life Safety, and Collapse Prevention. In our current building code we are only checking Collapse Prevention at 2/3 of the MCE event. It is assumed that with this, a Life Safety level will approximately be achieved at the DBE earthquake, and Immediate Occupancy will be approximately achieved at a "Frequent Earthquake". In other design documents (i.e. ASCE 41) Performance Levels are more explicitly defined and designed to. However, in the building code, adjustments to the expected performance levels is made through the modification of the Importance Factor "I". Therefore, essentially facilities with an I = 1.5 are designed implicitly to a higher performance level.

Hopefully that helps somewhat, that's about all I have time for...

 

[WiSEiwish]

Thank you for taking the time to reply.

From what I understand, tension only bracing is not allowed to be a part of the SLRS. Diagonal bracing must withstand both tension and compression loads. Does this rule out completely the use of these systems in a project located in a seismic design category E building, or does it mean that if tension only bracing is used, then it is neglected when analyzing any of the load combinations that include seismic loads? In other words, can I include the tension only bracing when running 0.9D + 1.6W + 1.6H and not when I'm running 0.9D + 1.0E + 1.6H? If so, is it good practice to do this, or should all lateral support systems conform to the seismic detailing requirements?

 

[sandman21]

tension only bracing is acceptable for OCBF (ordinary concentric brace frame) you may not use tension only with SCBF (special concentric brace frame). SCBF use the tension and compression to dissipate energy. In seismic zones, regards of which lateral load controls, wind v. seismic, seismic provisions, design and detailing shall be followed.