when to use Special Seismic Load from ASCE7-02? 2

[hippo11]

The Special Seismic Load of 9.5.2.7.1:

Em=omegaQE +- 0.2SdsD

is to be used "where indicated" in the code...

My question is, where do you use this Special Seismic Load, Em, besides 9.5.2.6.4.2 (collectors and collector connections)?

Do I use Em anywhere else besides for collectors? I hunted through the code couldn't find anywhere else besides 9.5.2.6.4.2...

Thnaks.

 

[hippo11]

Oh, okay...I think I see...so you define your system as "Structural Steel Systems Not Specifcally Detailed for Seismic Resistance" which is permitted for SDC A, B and C. Then you use R=3, omega=3 and Cd=3...alright. Yes?

thanks

 

[hippo11]

....and the Detail Reference Standard for Structural Steel Systems Not Specifcally Detailed for Seismic Resistance is simply the AISC regular manual, and you don't have to use 341...?...thanks

 

[blake989]

that's how I see it.

 

[slickdeals]

As I interpret the code, the braces themselves are designed for 1.1 Ry Ag and the connections are designed for the omega loads. The reason being that you want your brace to yield/buckle prior to your connection failure. This also enable energy dissipation under cyclic loading.

 

[hippo11]

which code, what yr?

 

[WillisV]

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=2671

 

[hippo11]

So we have E=rho*Qe+-0.2SdsD
Em=omega*Qe+-0.2SdsD


My next question...say you are in SDC D, with an OCBF structure, where you've designed your members for Em, your column base plate connection for Em, your connections for regular E....let's look at the foundation.

ASCE 9.5.2.7 says you can neglect 0.2SdsD for fdn o'turning.
IBC 1801 says you can take 75% of your quake load for fdn o'turning.

All that being said....now, for the foundation, do you do your strength design for Em or E?


How about when you check that under service-level loads you are < qa recommended by the geotech, and check that you have no net uplift...E or Em?

Thanks!

 

[blake989]

as for the foundation, in chapter 8 of AISC 341-02 it discusses this, it will kick you back to you building code for the soil (looks like you've already got that far).

When you say "strength design", I am assuming you're talking about reinforcing the foundation. IN MY INTERPRETATION all the omega factors and such are there to create a certain amount of pre-determined behavior/ductility in the "structural steel". With that being said, I ditch the omega for concrete design, but I've always retained Qe for overturning (i've never seen that exclusion in IBC, but I will look for it next time).

 

[hippo11]

But since the column and braces are being designed for Em, wouldn't it make sense to design the column base plate connection for Em, and therefore the foundation itself for Em, since Em is the force being transferred to the foundation by the column base plate connection? What's the point of designing your braces and columns for Em if your foundation will fail in flexure or bearing at Em anyway?

It just seems like you would include Em * 0.75 (IBC 1801) for your foundation reinforcing design, and include 0.7Em for your service-level pressure bearing check < qa.

No?