CAP ON OVERSTRENGHT FACTOR? 1

[jonnywalker]

I am designing a three-story house and there is a shear wall that sits over the garage so i have a discontinuous vertical irregularity. i am looking at an example in the Structural/Seismic Design Manual and they have a similar case and use an omega knot factor.

The uplifting force without the omega knot is high and if i use it there will be an uplift force greater than the total shear in the entire building.

Is there a cap? I cant seem to find one...but i hope there is.

 

[ishvaaag]

If there's not, there should be. Structures are designed to NEVER surpass the service level forces, and that includes any seismic forces in sight. So, with codes establishing the safety and reliabilty levels of structures, in more that too many times too many restrictions on feasible practices, and with a sound determination of the standing forces, it is clear that not only the actual strength of a properly sized member NEVER need be met by another (they are sized for the factored forces at reduced material strengths!), then how less for values exceeding their actual capacity. This is a coverall clause the more atechnical thing one can see.

If the codes want some level of safety, they demand what they want, precisely, point. No blankets just in case. The excuse of that if the supported member attains more than its capacity in overstrength the other must be there to stand is just stupidity: no member should attain such state in a proper design, they should stand about 2 to 4 times less than that anytime in the life of the building, seismic events included. If not, they need do their work and revamp the seismic forces at hand.

In short, as the code demands responsibility to the practicing structural designers, we are entitled to the more basic common sense in them.

 

[sandman21]

Structures are NOT designed to never surpass service level forces on any earthquake in sight, since we do not know the magnitude or location of the earthquake, buildings would need to be several times there current size.

To answer the OP question there is only one reduction that is for a flexible diaphragm building, ASCE Table 12.2-1. A discontinuous system is not an area where you want to save some material cost.

 

[ishvaaag]

sandman21,

to every purpose, earthquake effects' prevention through the application of some code, is tantamount to enforce that the structure is never surpass at service level SOME design earthquake, or if you want more precision, some set of tehm. The forces derived from the code are indicating us what forces we need cared for, and their effects. "El sismo normativo", we would say in spanish.

But that is only a notional concept useful to establish rationally some level of strength against earthquake, that we must adopt.

Of course, nature is not respecter of codes, has its own laws, and may deliver an earthquake blow very well above the provisions of any earthquake and non-earthquake code, but if it does so, from our designer's viewpoint, we having complied withe the seismic code, it must be our stance that it is the seismic code that is flawed, and, even, better, if we think so be the case beforehand, even outsmart it if it is in our power, for THAT is our responsability, to deliver safe buildings to use.

 

[InDepth]

Take a look at the EXCEPTION in ASCE 7-05 12.4.3.1. It pretty much limits the force level to the maximum amount the element can develop. i.e. for a MF, it would be the max shear developed by plastic hinging (assuming a hinge beam pattern). For Wood Shear walls, I'm sure you could calculate out a max force level that can be developed by the holdowns (axial hinging of holddowns with a material overstrength). Don't forget about the diaphragm design as well and account for an overstrength (or max) through it.

 

[InDepth]

Oh, and look at FEMA and NEHRP for explanations on the overstrength. The fundametal concept is to keep discontinous components elastic. If you plan on using a MF or other system below the garage, don't forget to use 12.2.3.1 for two stage analysis....and make sure you have displacment compatibility between the MF and wood shear walls on the same level. i.e. MF needs to be as stiff as shear wall to prevent torsional response.

 

[Strucasgdf]

There is a codified loophole for this, but there should not be. The current code is written so that you can take the maximum force the system can deliver, but that is called putting on your shoes before you socks and is POOR ENGINEERING!
These systems may be designed to remain elastic under design loads, but they are still just design loads. Most of the large earthquakes we have seen in California have been greater than the design earthquake loads as determined by ASCE 07. A vertical irregularity is a serious design consideration.
It is a cost issue that the Architect should be made aware of (or the client) and it may motivate them to allow you to move the wall so you can avoid irregularites.

 

[JAE]

InDepth - just to clarify, I think the "maximum force that can be delivered to the structure" as a limit is a limit directed upstream, so to speak. In other words, the shearwall strength "cap" would be, perhaps, the maximum force that the diaphragm could deliver to the shearwall, not the max. strength of the holddowns below.

Holddowns are downstream so to speak right? I know that the seismic force is really delivered from the ground up, but in using a static approach, the mass accelleration is looked at as if the force is brought into the shearwall from all the molecules of the building above it or beside it, not below it.

Just wondering what your thoughts are on this, thanks.

 

[frv]

ishvaaag-

Unless I'm misinterpreting your post, you have a flawed understanding of the seismic provisions.

This has been explained in other posts, but let me summarize as follows:

A structure is designed to withstand a design seismic event in the same way a car is designed to withstand a head-on collision. It is most certainly NOT intended to withstand the event in an elastic manner. Certainly, some parts of the SLRS are indeed designed to remain elastic, but that is so that the system as a whole can reach inelastic behavior (read "ductile").

A very important concept in seismic design is that a structure must be EITHER strong enough OR ductile enough to resist seismic movements (remember that there really is no such thing as a seismic "force"; rather forces are induced into the structure as a result of the displacement). Consequently, if your structure can accommodate the displacements (by proper and rigorous detailing), then the forces it must resist are greatly reduced).

 

[ishvaaag]

frv, thanks for your entry ... fortunately I understand it well and know that the instantaneous forces appearing during an earthquake far exceed those used for design. Now read of my post in contrary way: once determined the demand, our structure needs not to sustain more than that demand. That demand is of course with all the pertaining notes. Once you have that, earthquake or not, your structure is never to surpass such demands (or any other from any service level loading. That is more or less what I was attempting to say. It is seen more clearly in a non earthquake case, the assumption is that the live load is to be up to some value, even nailed in a plate, everything above is either an anomaly being dealt with by the intelligence of the code, and within it, having additional safety factors.

So if the seismic demand is well established, there should not be the requirement of the complementary note on overstrength ... but for the fact that those making the code have determined that they are not sure that the structure would be safe without such clause. In short, that determination of the demand would be short of the really required value if the clause was eliminated. And my view is that they should gain enough confidence on their methodoloy to eliminate what is a discordant note in what is a rational tradition of the mechanical era: that the structural elements need be apportioned to the demand. Yet this pertains to an older era: make it big, we don't know.

 

[sandman21]

The code can not address every situation. The code is not a cook book it is a guide, just because they add in additional factors of safety does not mean they lack confidence in there approach to seismic loads or any load. It is just an understanding that they nor anyone else can truly predict how a structure will act when discontinuous systems are used.

 

[ishvaaag]

That's true. But I stil think it is an add on, like the added eccentricity of masses. Couldn't we live without these inconveniences? Likely yes. Sometimes it only seems someone is wanting to make life more difficult, not as difficult.

 

[sandman21]

I found this one day looking for an explanation on "maximum force that can be delivered to the system"

“I believe the following comments from Jim Malley, Chairman of AISC Seismic Task Committee (TC9), will help answer your question (refer to item #2):

AISC TC9 recognizes that the means of determining the maximum force that can be delivered has been left unspecific. There are a number of means to determine this force, which all may be appropriate in different circumstances. They include:

1) Performing a pushover analysis and determining the load on the connections at the maximum capacity of the frame.

2) Determining how much force can be resisted before uplift of a shallow foundation (spread footing). Note that the foundation design forces are not required to resist more than the code base shear level. This is not typically applied for a deep foundation since the determination of when uplift will occur is not easy to determine with any accuracy.

3) Performing a suite of inelastic time history analyses and enveloping the connection demands.

Some would argue that applying Omega sub zero to the design forces from the code base shear is enough to satisfy this provision in all cases. This was allowed in the 1992 Seismic Provisions (and other seismic provisions), but TC9 removed it from the 1997 Provisions because of the concern that a global overstrength factor like the Omega sub zero factor would not be appropriate to use on a local critical demand like the connections in a braced frame. Individual connections may see forces much higher than this in order for the frame to achieve it's maximum overall capacity. This type of approach may be more appropriate for systems with very limited ductility expected, such as moment connections in metal buildings that will likely buckle elastically well before reaching the Mp of the members, thereby limiting the load to be delivered to the connections. This would generally not be the case for SCBF's however, since these frames are configured and designed to buckle the compression braces in the inelastic range, and yield other braces in tension.

Calculating the maximum connection force by one of the three methods noted above is not a common practice on design projects. In some cases, such an approach could result in smaller connection demands. But, from a conceptual basis, since the character of the ground motions is not known to any great extent, it is unrealistic to expect that such forces can be accurately calculated. All three approaches rely on an assumption of the distribution of forces which may not match reality (approach #3 above probably being the best estimate, but also the most calculation intensive). TC9 believes that in most cases providing the connection with a capacity large enough to yield the member is needed because of the large inelastic demands placed on a structure by a major earthquake.

Regards,

Sergio Zoruba, Ph.D.
Senior Engineer
American Institute of Steel Construction, Inc.
866.ASK.AISC”

 

[ishvaaag]

Thanks, sandman 21.

In the end it is seen that TC9 at least provides some alternative ways to determine the force/displacement solicitations through some form of calculation. It is not on the more common response spectrum analysis way, but some ways there are, anyway. It is also curious that one way on scaling effects from one RSA determination is not devised, whilst one based in a presumably, lesser set, of time historiy checks, is.

Also, it seems reasonable the opinion on that the requirement of the connection be able to sustain the yield of the member for "major earthquakes". But for zones where are expected earthquake events of lesser intensity the requirement being made mandatory (even if by choice of structural system) might become unwarranted.

Also, other than the specified ways above, are not any ways to proceed more automatically for the design? My view is that a set of ways of calculations were devised along decades, one of them -take awat equivalent static- for ordinary buildings, became predominant (RSA) and, along the way, some milestones of addon safety were added such

Minimum base shear
Geometrical and structural system requirements
Overstrength factors
Additional eccentricity
Additional ways of checking the structure

These things may well -in the precise manner specified- be convenient requirements for safe structure, but leave one unsatisfied from the viewpoint of integration of the analysis and design, you have analyzed it and it is like you have not, and have to start again. This makes difficult the lives of those designing the softwares and the structures, we do it -professionally- in the interest of people, but in my view it is clear the whole process might be tweaked to more complete integration.

May be just a tweaked basic process of designing against earthquake will be able to provide the socially expected and affordable warrant against earthquakes, to negligible difference with the current setup.