Moment Magnification - Approximate 2nd Order Analysis 2

[RFreund]

I think I am getting confused between steel approximate second order analysis and concrete. Codes: AISC360 and ACI318

In Steel:
"Braced Frame or Nonsway":
Required Axial Strength, Pr=Pns
Required Moment Strength, Mr=B1*Mns
"sway or moment frame":
Required Axial Strength, Pr=Pns+B2Ps
Required Moment Strength, Mr=B1*Mns+B2*Ms

In Concrete:
"Braced Frame or Nonsway" and columns are slender:
Required Axial Strength, Pr=Pns
Required Moment Strength, Mr=dns*Mns (Section 10.10.6)
"sway or moment frame":
Required Axial Strength, Pr=Pns+Ps
Required Moment Strength, Mr=Mns+ds*Ms (Section 10.10.7)

Is this correct?
Differences:
1. Axial load in concrete is not magnified in concrete.
2. For a sway frame, steel design uses nonsway magnifier B1 and sway magnifier B2. Concrete uses only a nonsway magnifier for nonsway and only a sway magnifier for sway.

Thanks!




EIT

 

[DST148]

Ref - ACI 318-05: In concrete design it is always the moment that is magnified, the axial load on a compression member is not altered.
Section 10.12, Nonsway frames - Amplification of moments caused by displacements of the braced points is negligible and so ignored(P-D effects). The moment magnification factor dns is to account for member curvature effects (P-d effects). It is a local factor and is required to be calculated only if klu/r for a subject column exceeds the value given by eqn. (10-7).
Section 10.13, Sway frames - Amplification of moments caused by displacements of braced points (P-D) effects is accounted for by calculating ds, which is a global factor and applied to all the columns of a given sway story. Amplification of moments to account for member curvature effects still needs to be accounted for compression members with lu/r greater than the value given by eqn. (10-19). This is done by multiplying the amplified moments at the two ends of the column(P-D effects) further by dns(P-d effects).

Ref AISC 360-05: B1 is an amplifier to account for member curvature effects caused by displacements between the brace points(P-d effects) and B2 is an amplifier to account for second order effects caused by displacements of braced points(P-D effects). B1 is similar to dns in concrete and is calculated for an individual column, B2 is similar to ds in concrete and is applicable to all the columns of a given sway story.
Nonsway Steel Frame - B2 = 1.0 (global), B1 to be calculated for each individual column. (Axial load is not altered)
Sway Frame - B2 > 1.0 (global), B1 to be calculated for each individual column. ( Axial load is altered)

 

[RFreund]

Thanks DST,

Some how I missed this "Amplification of moments to account for member curvature effects still needs to be accounted for compression members with lu/r greater than the value given by eqn. (10-19)"

I will look on Monday. That makes sense though however in my Nawny text they don't make this very clear IMHO.

EIT