33% Reduction in Seismic Design

[RUS777]

I am in the Middle of seismic analysis for a plant that we are building in Peru.

Seismic activity is very high over there, I am getting Cs value of roughly 35% with the 2/3 reduction of the spectral acceleration parameters. With out the 2/3 reduction I am getting a Cs value of roughly 51%.

From what I understand is that 2/3 reduction is used to make the building more ductule, allowing deformations into inelastic region of steel.

Structures that I desgn are more of an equipment platforms (aggregate crushing plants). I would not want to see any inelastic deformation at all, so my guess is that I need to design everything with out the 2/3 reduction.

Any thought on this?

Thank you in advance.

 

[DWHA]

The best description that I have ever came across is attached. Basically we design for the MCE and that is beyond what a typical structure would normally see.

 

[DWHA]

Sorry, the attachment didn't load

 

[DWHA]

From Earthquake Engineering Handbook (Chen & Scawthorn)

The
NEHRP Provisions
are intended to provide a tiered series of performance capabilities for structures, depending on their intended occupancy and use. Under the NEHRP Provisions, each structure must be
assigned to a seismic use group (SUG). Three SUGs are defined and are labeled I, II, and III:
• SUG-I encompasses most ordinary occupancy buildings, including typical commercial, residential,
and industrial structures. For these facilities the basic intent of the
NEHRP Provisions
, just as with
earlier codes, is to provide a low probability of earthquake-induced life safety endangerment.
• SUG-II includes facilities that house large numbers of persons, persons who are mobility impaired,
or large quantities of materials that, if released, could pose substantial hazards to the surrounding
community. Examples of such facilities include large assembly facilities, housing several thousand
persons, day care centers, and manufacturing facilities containing large quantities of toxic or
explosive materials. The performance intent for these facilities is to provide a lower probability of
life endangerment, relative to SUG-I structures, and a low probability of damage that would result
in release of stored materials.
• SUG-III includes those facilities such as hospitals and emergency operations and communications
centers deemed essential to disaster response and recovery operations. The basic performance
intent of the
NEHRP Provisions
with regard to these structures is to provide a low probability of
earthquake-induced loss of functionality and operability.
In reality, the probability of damage resulting in life endangerment, release of hazardous materials, or
loss of function should be calculated using structural reliability methods as the total probability of such
damage over a period of time [Ravindra, 1994]. Mathematically, this is equal to the integral, over all
possible levels of ground motion intensity, of the conditional probability of excessive damage given that
a ground motion intensity is experienced and the probability that such ground motion intensity will be
experienced in the desired period of time. Although such an approach would be mathematically and
conceptually correct, it is currently regarded as too complex for practical application in the design office.
Instead, the
NEHRP Provisions
design for desired limiting levels of nonlinear behavior for a single
design earthquake intensity level, termed
maximum considered earthquake
(MCE) ground shaking. In
most regions of the United States, the MCE is defined as that intensity of ground shaking having a 2%
probability of exceedance in 50 years. In certain regions, proximate to major active faults, this probabilistic
definition of MCE motion is limited by a conservative deterministic estimate of the ground motion
intensity anticipated to result from an earthquake of characteristic magnitude on these faults. The MCE
is thought to represent the most severe level of shaking ever likely to be experienced by a structure,
though it is recognized that there is some limited possibility of more severe motion occurring.
Structures categorized as SUG-I are designed with the expectation that MCE shaking would result in
severe damage to both structural and nonstructural elements, with damage perhaps being so severe that
following the earthquake, the structure would be on the verge of collapse. This damage state has come
to be termed
collapse prevention
, because the structure is thought to be at a state of incipient but not
actual collapse. Theoretically, SUG-I structures behaving in this manner would be total or near total
financial losses, in the event that MCE shaking was experienced. To the extent that shaking experienced
by the structure exceeds the MCE level, the structure could actually experience partial or total collapse.
SUG-III structures are designed with the intent that when subjected to MCE shaking they would
experience both structural and nonstructural damage; however, the structures would retain significant
residual structural resistance or margin against collapse. It is anticipated that when experiencing MCE
shaking, such structures may be damaged to an extent that they would no longer be suitable for occupancy,
until repair work had been instituted, but that repair would be technically and economically feasible.
This superior performance relative to SUG-I structures is accomplished through specification that SUG-III structures be designed with 50% greater strength and more stiffness than their SUG-I counterparts.
SUG-II structures are designed for performance intermediate to that for SUG-I and SUG-III, with
strengths and stiffness that are 25% greater than those required for SUG-I structures.

 

[RUS777]

So basically 2/3 reduction is used because if structure is designed for an applicable MCE then design strength is far greater that what is needed based on the Seismic Use Group (SUG).

So in other words, SUG-I and SUGII could use a 2/3 reduction to make design more economical and SUG-III should be designed for a full MCE capacity. But using a 2/3 reduction designer should assume that major structural damage will be incured during MCE if it happens.

 

[bookowski]

From: "DEVELOPMENT OF SEISMIC GROUND-MOTION CRITERIA FOR THE ASCE 7 STANDARD"

The major change in the ground-motion criteria published in the 1997 NEHRP seismic provisions (BSSC, 1998) was a noteworthy departure from the previous (1994) edition (BSSC,
1995). The Maximum Considered Earthquake (MCE) term was introduced to represent ground motions with a 2% probability of being exceeded in 50 years (average return period of
approximately 2,500 years), except in certain higher seismic regions near active faults, where deterministic estimates governed the MCE design motions. The design philosophy was to
preserve life safety and prevent collapse if the MCE ground motion occurred. Design ground motions were set at 2/3 of the MCE ground-motion level, with the reasoning that any structure
designed to the new seismic provisions, had a minimum margin against collapse of 1.5 (BSSC,2004b).

 

[DWHA]

Rus
In the end it is your call on how you design the structure. From your statement about the structure, it does not appear to have any (or minimal) life safety concerns. Therefore designing the structure to without utilizing the reduction factor would be a significant overdesign. Not bad thing, but be sure that the client is okay with this, as it is his money. And the structure will be much more expensive.

If you follow the code, you are using an accepted and approved method of design. You are also legally protected.

 

[RUS777]

Thank you all for your input.

Our structures are already pretty beefy to cope with violent vibrations that are introduced from crushing 2 food diameter rocks.

I just modeled my structure in Risa 3D using 51 percent of the seismic weight of the building (DL + 0.25LL) and sturcture remained pretty much intact, except for 2 angle brace members, which I was thinkig of making a double angles anyway.

Now I might use 2/3 reduction for checking some of the critical connections but I have a feeling that they will pass ether way.

Once again thank you for the responces. Now it is clear in my head why and where 2/3 reduction is used.

Should have asked all these questions in shool when I had a chance.