Do I need to use the amplified seismic load 1

[dcceecy]

I am designing some buildings with seismic load resistant systems of Special steel moment frame, Special concentrically building frame, and cold-formed steel stud walls with flat strap braceing.

I am not quite sure when I should use the "amplified seismic load determined by Omega factor" to design the members and connection of those systems?

The ASCE-7 just specified that for the collector member, splice and connections, the load combinations with Omega should be used. (I am not so familiar with ASCE-7)

I have some questions

For special steel moment frame, do I need to use the amplified seismic load to design the connection between column and base plate, base plate, base plate to concrete footings (anchor bolts), and footings?

For special concentrically building frame, do I need to use the amplified seismic load to design the connection between base plate to concrete footings (anchor bolts), and footings? or should I use the the expected yield strength of the brace and compressive strength of braces?

For cold-formed steel stud wall with flat strap x-braceings, do I need to use the amplified seismic load to design the top chords, flat strap, endposts, connections between flat straps and endposts, and endpost holddown?

I think there should have some complete design examples to follow instead of finding out by myself.

Any suggestions and comments.
Thanks.

 

[EngineeringAdam]

You'll want to read the AISC 341-05 (seismic provisions for structural steel buildings).

In it you will find, in section 8.5, the requirements for column bases. In short, for moment frames you can use either the omega load for the connection between column and foundation or the expected yield strength for the column to hinge. For braced frames you use the expected yield strength of the bracing connections. You do have to design all the elements involved as you mention - base plate, anchors, footings.

You do not need to design the cold-formed top chords, flat straps, endposts, etc with the amplified omega load. You WOULD need to use the omega load on the SUPPORTING member if your cold formed shear wall hold downs were discontinuous to the foundation. In this case you would size you hold downs to the normal load and design the supporting member (supporting the shear wall) with the omega load and follow the forces to the foundation. This is to encourage failure in the seismic load resisting system (SLRS) instead of your gravity member - so you can see that increasing the strength of your hold down and gravity member simultaneously isn't required.

For collectors, splices, and their connections to the seismic force resisting element you do need Omega forces (unless you have a light framed structure). This can be thought of in the same way as the discontinuous hold down case mentioned above. In order to insure ductile failure in your seismic load resisting system (SLRS), the code requires over-strength of the collectors (etc) that deliver the load to you SLRS. It is all done to insure failure occurs in the SLRS.

SEAOC has a few design guides that you should check out called "structurel/seismic design manual", there are three volumes.

Also, I should note that you should consider getting help, whether a mentor, or outside peer review to guide you if you don't have experience in such design.

 

[dcceecy]

Thank you for your reply.

For special steel moment frame ,
Do I need to use the load combination with Omega to check if the soil bearing capcity is enough or not?

From my understanding, basically, for special steel moment frame, we want the yielding to happen only in the protected zone of beams and keep everything else (include base plate, anchor bolts, footing) elastic even under seismic load = Omega times of design seismic load.

For special concentrically building frame,
Do I need to use the expected yielding strength to check if the soil bearing capcity is enough or not?
From my understanding, basically, for special concentrically building frame, we want the yielding happens only in the braces and keep everything else (gusset plate, beams, and columns) elastic even under seismic load = Omega times of design seismic load. We also want that the braces connections (also include base plate, anchor bolts, and footings) to be designed at the maximum possible tensile strength of braces.

For the CFS x-bracing shear wall, I am thinking of use some Simpson straps as holddowns and use Simpson Titens as anchor bolts to concrete footing. Does that mean I have to design the anchor bolts and concrete footing with the Omega load? If the shear walls are also bearing walls. Does that mean I have to design the endpost with the Omega load?

I also thought CFS x-braces is similar to special concentrically building frame, we want yielding to happen in braces. So we have to design x-braces, endposts and top chords with Omega load, and design the holddowns, anchor bolts and footing to the expected yielding strength of x-strap.

I am studying these stuff. Any comments will be appreciated.

Thank you.

 

[EngineeringAdam]

As far as I understand it you do not need to design the soil bearing capacity against an Omega load.

You don't need to design the brace frames, moment frames, or even CFS brace walls with the omega factor by default - the way you design the connections of these structural elements is how you force yielding to occur in a desirable place.


The Omega factor only comes into play in certain situations - e.g. collectors, discontinuous seismic resisting systems to the foundation, and certain connections to he foundation itself.


If you are just using the omega factor for everything you are doing it wrong, although conservatively wrong.

 

[jdgengineer]

I must disagree with EngineeringAdam about using the Omega factor for everything would be "conservatively wrong". The intent of the omega factor is to ensure that certain elements do not fail before the yielding mechanism. By using the omega factor for everything you would essentially eliminate the intent of the omega factor and would instead be designing your building for higher forces R / omega. In my opinion this is not the intent which is to keep the load path intact to deliver the forces to your yielding mechanism.

If the seismic forces are higher than expected, elements could yield in unexpected areas which could significantly compromise the ductility of the building during an earthquake. This comes up with single diagonal braced frames which is why their use is discouraged for seismic design.

 

[EngineeringAdam]

I must disagree with jdgengineer about the application of the omega factor.

However, while I do agree that the intent of the omega factor is to encourage yielding to occur in specific desirable locations, like I already mentioned in my previous posts, I don't agree that it isn't "conservatively wrong" it to apply it globally.

Let's use the single diagonal brace example mentioned. If you have to apply an omega factor to a single diagonal brace because you don't have alternating braces along the same line, you don't have to then apply the omega factor twice to the collectors delivering load. The code allows this, although you could argue that the fuse is now possibly within the collector and not the brace - but I would argue that this is acceptable since a single diagonal brace capacity to resist compression forces after in-elastic deformation has occurred is extremely low (30% of original capacity). The collector failure can be more desirable than a brace failure because a brace failure can cause a story collapse mechanism. While not ideal, at least there can be secondary paths for load to collect if the main collector was to fail.

Also, Omega factored forces are allowed to be substituted for expected yielding forces in other places as well.

At moment frame column bases you can elect to use omega force levels for shear and flexural strength instead of the higher expected yielding forces for design of the column bases, including their attachments to the foundation. In this case your fuse might be at the connection base. But the code still allows this.

I also point out that the omega factor is 2 or 3 for brace frames or MFs. The load increase is massive and ductile detailing requirements would still need to be met in the main SLRS.

Lastly, I would like to point out that in the ideal case, non-omega loads to a brace frame SLRS with the omega load applied to the collectors, system failure isn't guaranteed to occur in the brace frame. For example, the expected yielding strength of a brace (where failure is supposed to occur due to careful connection design and detailing) is usually MUCH higher than the force that is delivered according to the elastic analysis - in this case your collectors would still be the fuse since the collectors are designed to the elastic force levels and the brace is designed to fail at the much higher expected yielding strength of the brace.

I think applying the a global omega is very cost ineffective, but still conservative for those reasons.

 

[EChristensen]

Jumping this train back on track, if it's not too late...

For the CFS shearwall. Use AISI LATERAL (2004) or AISI S213 (2007), depending on your code of record.

Seismic detailing requirements for both are essentially the same and are 'tricky'. The chord studs and anchorage are designed for the non-amplified load BUT you do need to check the nominal capacity against "loads the system can deliver, but not to exceed the amplified load." that's the tricky part. Connections of chords and collectors are the same trickiness.

The Lateral Design spec also specifically states that the foundations do not need to be designed for the amplified seismic loads.

As was mentioned earlier before this thread derailed a bit, and as is done in AISC 341 for hot-rolled steel, the special seismic detailing is mostly ensuring failures don't happen in the connections but rather are out in the members where things happen in a more ductile fashion.

Finally AISI publishes design guides (similar to AISC) and the CFSEI (Cold-Formed Steel Engineers Institute) also has the same design guides as well as research & tech. reports. AISI and CFSEI publications are all available from the Steel Framing Alliance (which hosts AISIs online store). All helpful resources, a lot of them free downloads!

I also seem to remember seeing a paper from Simpson Strong-Tie regarding the design of CFS shearwalls which may include a design example.

Hopefully this is all helpful and not too late.

 

[dcceecy]

In case of x-strap braced stud wall, it seems the x flat straps do not have to be designed for amplified seismic load.
If the strap is not so over designed, the maximum load the system can deliver may be based on the nominal capacity of the strap, which may resulted in smaller load on anchorage or endpost than using the amplified seismic load.
Is my thinking correct? Thanks.

 

[EChristensen]

I must apologize, I got lost in the "to over-design or not over-design" discussion; I forgot you are working with x-braced shear wall.

Yes, you are on the right track. Design the straps for non-amplified seismic load.

However, you will need to know the amplified seismic load when you're designing the connections for the strap, boundary members and collectors, as well as the chord studs, strap anchoring device (boot) and the hold-down anchor (not the foundation). The available strength of the strap, boundary member and collector connections, as well as the nominal capacity of the chord studs and hold-down assembly need to be greater than the expected yield of the gross section of the strap (Ag*Ry*Fy) with the amplified seismic load being the upper limit on the force for all these items. (I'm also assuming you are in the US)

All this in its gory detail (just one page, not that gory) can be found in AISI S213-07 Section C5.2. If you can get your hands on that specification, that would be best.