Overstrength Factor Applicability Ωo

[BoilerEng]

I've been searching the web for a sort of comprehensive list of what elements get designed for the Overstrength Factor, Ωo, of ASCE 7-10 and have been unsuccessful. I understand how to apply it, just not particularly to which items.

I was hoping I could get some input as to what items to pay attention to that require designing with the Overstrength Factor.

I know it is material specific and things like connections, anchor bolts, etc are typically designed for it, but this is usually just a small blurb (section) in the material-specific code and I don't want to miss it when taking the exam.

If anybody has any input or if we can get a list of replies going, that would be great!

I'll start with some sections I came across recently:

ACI 318-11:

21.3.3.1(b): ØVn of beams resisting earthquakes shall not be less than max shear obtained from design load combos including E, with assumed to be twice that prescribed. This is effectively and overstrength factor of 2.

21.3.3.2(b): ØVn of columns resisting earthquakes shall not be less than the maximum shear obtained from design load combinations that include E, with E increased by Ωo

*Note - both of these sections have an (a) with shear associated to nominal moments of the beams or columns, respectively.

D3.3.4.3(d) and D3.3.5.3(d): Where anchors are required to be designed for seismic effects, shear and tension can be satisfied by designing the anchors using overstrength factor.

 

[BoilerEng]

D3.3.5.3(d) should actually be D3.3.5.3©

 

[TehMightyEngineer]

Attached are my notes for ASCE 7-05 on the overstrength and redundancy factor. Hope it helps.

Overstrength and Redundancy Factor.pdf

 

[BoilerEng]

Thanks! That's definitely helpful

 

[Andy Lin]

Someone asked me a similar question last year and the following was my answer (based on prior code though - current code should be similar).

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Overstrength Factor ([SIZE=11pt]Ω[/SIZE]0)

In general (i.e. for all types of structures), overstrength applies to the following based on ASCE 7-05:

• 12.2.5.2 Foundation and other elements at the base of a cantilever column.
• 12.3.3.3 Elements supporting discontinuous walls or frames
• 12.10.2.1 Collector Elements (Exception: light-frame shear walls, i.e. plywood shear walls)
  
• 12.13.6.4 Batter Piles
• 12.13.6.5 Pile anchorage
• 12.13.6.6 Splice of pile segments

The collector/drag is in bold because it's most common.


25% Increase Due to Irregularity (12.3.3.4)

Now, besides the omega naught, you also want to read section 12.3.3.4 because due to certain irregularities, some elements (e.g. diaphragm connections) need to be designed for an increase in force of 25%.


Material Specific

Now for the actual design of lateral system, usually the reference code tells you. Sorry I haven't done concrete and masonry for awhile so I'll give you an example in steel.

In AISC Seismic Provision, you want to watch out for the term "amplified seismic load". For example, here a few things in general:

• Section 8.3: When DC ratio is > 0.4 without consideration of the amplified seismic load, you want to then design the column with [SIZE=11pt]Ω[/SIZE]0.
  
• Section 8.4a: Welded column splices.

Now for system specific requirement, you have to look through them to know because sometimes it's not necessary "required" but more of an upper limit for design load.


For example, check out 14.4 Exception (b) for Ordinary Concentrically Braced Frames: "Bracing connections max design force doesn't need to exceed the amplified force."


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[McEngr]

AISC 341-10 essentially requires you to design all columns for overstrength now. At least for me, on the west coast, if I'm designing an AISC 341 system, I'm using overstrength columns...

This was a big deal in '05 where you had to be careful. Now, it's more conservatively using the overstrength on almost all conditions. Obviously, you can still design to the maximum force delivered. I've tried to argue that a salty engineer could design on OCBF and work down the anchorage and column design requirements based on a 'tip-over' analysis for the foundation, but many (if not most) engineers want to stay conservative and not do that (at least in my office)...

 

[Andy Lin]

AISC 341-10 essentially requires you to design all columns for overstrength now. At least for me, on the west coast, if I'm designing an AISC 341 system, I'm using overstrength columns...

This was a big deal in '05 where you had to be careful. Now, it's more conservatively using the overstrength on almost all conditions. Obviously, you can still design to the maximum force delivered. I've tried to argue that a salty engineer could design on OCBF and work down the anchorage and column design requirements based on a 'tip-over' analysis for the foundation, but many (if not most) engineers want to stay conservative and not do that (at least in my office)...

Nice - good to know. Thanks!

 

[BoilerEng]

AISC 341-10 essentially requires you to design all columns for overstrength now. At least for me, on the west coast, if I'm designing an AISC 341 system, I'm using overstrength columns...

This was a big deal in '05 where you had to be careful. Now, it's more conservatively using the overstrength on almost all conditions. Obviously, you can still design to the maximum force delivered. I've tried to argue that a salty engineer could design on OCBF and work down the anchorage and column design requirements based on a 'tip-over' analysis for the foundation, but many (if not most) engineers want to stay conservative and not do that (at least in my office)...

When you say to design columns for overstrength - do you mean the applied earthquake loads with overstrength factor? And when you say you can still design to the maximum force delivered, are you referring to the expected tension and post-buckling strength of your braces in an eccentrically braced frame?

Am I reading the code correctly in that for an eccentric frame, I first design the braces for E times redundancy factor. Then I design the rest of the system according to expected tension and compression of the braces. But I can also look at the applied earthquake loads times overstrength and design columns for that instead? Basically whichever is lower?

 

[McEngr]

D1.4a Required Strength

^^This is the section I'm referring to. You use the amplified seismic load, but this is a lower bound. In almost all cases for SDC D thru F, your post-buckling forces will exceed this. However, both should be checked. This comparison is obvious of you look at some of Rafael Sabelli's comments in the SEAOC Seis Des volumes. The cumulative effect of eccentric braces on multiple floors will obviously be many times more than the sum of story shears divided by the bay distance: F3+F2+F1 divided by L will be less than say 1.1RyFy of 3 braces at the base.

If you look at part 1 of D1.4a, it says "load effect resulting from the analysis requirements..." - basically you need to check both. The point of my answer was to answer the overstrength question.

 

[BoilerEng]

D1.4a Required Strength

^^This is the section I'm referring to. You use the amplified seismic load, but this is a lower bound. In almost all cases for SDC D thru F, your post-buckling forces will exceed this. However, both should be checked. This comparison is obvious of you look at some of Rafael Sabelli's comments in the SEAOC Seis Des volumes. The cumulative effect of eccentric braces on multiple floors will obviously be many times more than the sum of story shears divided by the bay distance: F3+F2+F1 divided by L will be less than say 1.1RyFy of 3 braces at the base.

If you look at part 1 of D1.4a, it says "load effect resulting from the analysis requirements..." - basically you need to check both. The point of my answer was to answer the overstrength question.

Makes sense. Thanks!

 

[McEngr]

You're welcome! The fact you are asking the question means you're far along into the more advanced PE and SE topics. Good luck!

 

[JunZ]

check the new ASCE7-10 commentary, I had a argument with local DBI, and she/he refused to read ASCE7-10 commentary. 

 

[kevo_55]

Commentary isn't cited in chapter 35 of the 2012 IBC.....