AASHTO 2010 R-Factors

[smjohns78]

I am trying to study for the SE and the examples and commentary I am finding don't seem to be consistent with each other and have confused me as to the intent of the code. I am hoping that someone might have some insight to the use of R-Factors

Based on the examples and AASHTO 2010 my understanding is that for Seismic Zone 2 you determine elastic forces and modify all demands, shear/axial/bending, by the appropriate R-factor.

For Seismic Zones 3&4. The code states that you should use the lesser of the demand based on either 3.10.9.4.2 or 3.10.9.4.3. So for example, say my controlling demands (factored but unmodified) based on an RSA analysis were,

Mu = 6000 k-ft
Vu = 600 kips
Pu = 1000 kips

If I chose a system with an R-factor of 2.0 then per section 3.10.9.4.2/3.10.9.3 my demands would be,

Mu = 3000 k-ft
Vu = 300 kips
Pu = 500 kips

Then say per section 3.10.9.4.3c I calculate forces per the plastic hinging moment using an overstrength factor of 1.3 I get the following forces,

Mp = 4000 k-ft
Vp = 350 kips
Pu = 450 kips

My interpretation would then lead me to believe that my design forces per 3.10.9.4.1 are

Mu_controlling = 3000 k-ft
Vu_controlling = 300 k-ft
Pu_controlling = 450 k-ft

Any insight would be appreciated.

 

[maneesha0702]

I just wanted to know how you did on this exam and how you prepared for it.

 

[VoyageofDiscovery]

This may be a little late for your exam.

The intent of applying R factor to elastic forces is to capture inelastic or plastic behavior if we allow (design) the structure flex, yield and absorb energy safely without a sudden failure. Thus do not apply modification the R factor to shear or axial forces (therefore R=1.0). Note that R=1.0 for moment, shear and axial in terms of any capacity protected elements.

HTH

VoD