Overturning Moment OMF vs SMF 3

[Penpendrum]

Good day .

I have been wondering why is it that the overturning moment for OMF building is far greater than the SMF building although they would have approximately the same mass?

Code is UBC 97 , and ASCE 7-14

Is there a limit for the overturning that would give the same results to both desired design?( correct me for my assumptions)

 

[HTURKAK]

1- Because response modification factor (R) is different .. R= 8 for SMF ,While R= 3 for OMF etc..
2- You are expected to satisfy the minimum requirement. For ASCE 7-14 ( 12.8 EQUIVALENT LATERAL FORCE (ELF)

Look to your local code for minimum requirements with ELF analysis.

 

[KootK]

I have been wondering why is it that the overturning moment for OMF building is far greater than the SMF building although they would have approximately the same mass?

One way to think of it is like this:

1) Above the base level, there is no such thing as an externally applied seismic force. Rather, there is only an imposed, seismic displacement which generates internal forces.

2) The higher R-Values used for the more ductile systems make a portion of the imposed displacement plastic such that it does not generate additional internal forces.

3) Since your overturning moment is the aggregate of all of the internal force actions above that contribute to it, [#2] means less overturning for the higher R-value systems.

Is there a limit for the overturning that would give the same results to both desired design?( correct me for my assumptions)

Sort of. In most cases, capacity design principles dictate that the foundation remain in tact while the plastic hinge mechanism above is developed. Different standards treat this in different ways such as:

1) Designing the foundation for over strength level loading.

2) Insisting that the foundation remain elastic through the development of the plastic hinging mechanisms.

These things generally mean that the foundation will be pushed closer to an OMF design even if the structure above is a higher ductility system.

 

[Penpendrum]

@HTURKAK thanks for the advise
@KootK sorry i forgot to mention that my main concern is the design of the foundation .. now that you have mentioned it.

In some structures that have large height to width ratio .. if using an OMF category for R value .. the foundation would rather tip over than using the value of R of the SMF..

From what i have understood in your statement it would be wise to design the foundation to OMF category?Or there is something else..? (Using ASCE)

 

[JoshPlumSE]

Penpendrum -

To reiterate what KootK was saying, but maybe say it in a different way. What happens during an earthquake? The ground moves. How does the structure respond to that?

An extremely flexible structure (think base isolation) allows the ground to move under it, and it just deflects without creating much in the way of inertial forces in the structure.

Now, let's compare and SMF design to a design that has R = 1 (meaning a purely elastic design). The SMF is allowed to resist the inertial forces through inelastic deformation. That means the ENERGY input caused by inertial seismic forces is dissipated through these inelastic defelections, rather than being purely transmitted through the structure and down into the foundation as it would be in a purely ELASTIC deflection like you'd see in an R=1 structure. Makes sense?

The same is true for a SMF vs and OMF. It's just that we're comparing different levels of inelasticity. The OMF is not detailed in a way that it can survive the kind of inelasticity that an SMF will. Therefore, more of the inertial forces will be converted into elastic strain energy and must be resisted by the structure and carried down into the foundation.

Now, there is an out or a caveat to this.... For OMF's, I believe there is still the "maximum force that can be transmitted by the system" caveat. And, some people will use foundation overturning as a way to reduce the maximum forces that can be transmitted by the system. I'd be cautious about doing this, personally. I have a good understanding about how to increase ductility of steel and concrete members. I don't have a good understanding about how to do that with soils / foundation interaction and that makes me nervous to do so. If I were to do it, I'd probably want to talk to a geotech expert about it and get his opinion on the subject.