Interpretation of the intent of NZS 3101:206 and ACI 318-02

[youngstructural]

Hello All;

I appologize in advance to our American friends; I know that the ACI 318 has this procedure only because the New Zealand code [NZS 3102:2006] states that it is taken from ACI 318-02. I am looking for some guidance on the application of cl. 11.3.5.1.2 in NZS 3101:2006. Clause 11.3.5.1.2 “Design moment and P-delta effects – simplified method” uses a combination of applied moment and axial load in determining the total moment for the calculation of the design moment. Considering a typical multi-storey tilt-up wall panel, which of the following procedures meets the intent of the code?

1) Calculate the moment on the wall from the eccentricity of bearing point for floors framing into the wall, apply this as Ma* and then calculate N* (design axial load on the wall) which must include the floor and roof loads, apply ?u term to N* and sum the result with Ma* find the total design moment. (Unlikely, as this double the effect of the floor loads)

2) Calculate Ma* from the floor loads with their bearing eccentricity, and consider only the wall self-weight in calculation of N* for this clause, apply ?u term to the resulting N*, and sum the two to find M*.

3) Ignore bearing eccentricity of the floors and roof, and consider only atypical exterior loads (perhaps beams framing into the wall, or a torsional loading such as a motor starting up) as being Ma* loads. This would mean that the calculation of the moment on the wall would be independent of the actual eccentricity of loading on the wall, and based purely on an assumed parabolic deflected shape of the wall.

Which of these procedures, if any, matches the intent of the code? Reading the code to the letter, it would seem that number 1 is the correct answer, however this results in quite high moments, since it in effectively doubles the effect of the floor and roof loads. I am tempted to approach the problem as per number 2, however this would mean using a reduced N* value, which is not what the code says to consider.

Thank you in advance for your help and opinions...

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

1) Calculate the moment on the wall from the eccentricity of bearing point for floors framing into the wall, apply this as Ma* and then calculate N* (design axial load on the wall) which must include the floor and roof loads, apply ?u term to N* and sum the result with Ma* find the total design moment. (Unlikely, as this double the effect of the floor loads)

I'm not sure what the ?u term to N* means. Is that a moment magnification factor?

Why do you see that as doubling the effect of floor loads. The floor loads would have their axial component AND they would have resulting bending moments in the wall panel that would be combined with wind/seismic moments per the code required load combinations.

 

[Zhuge]

Having read these 3 points you stated above, actually they are not separate points, they are related to each other.
Step 1, you need to know the eccentricity of the bearing point to the centre of the wall. Then apply Gravity load from the floor/roof multiply by the eccenticity = Ma

Step 2,Combine Ma with M* (design moment without considering the eccentricity). Then You'll get final design moment with the eccentricity being considered.

Step 3,Basically another way of modelling your structure, instead of adding up Ma + M*, you can model your wall without considering the eccentricity, but you need to apply Ma on it.

 

[youngstructural]

Thanks for the replies... I should not have pasted a greek character into the forum, the ?u is certainly confusing the issue.

DELTA u (or ?u in my original post) in an imposed eccentricity calculated by the code to simulate the effect of the wall deforming parabolically. Thus, when multiplied by N* (the factored axial load in the wall) we get a moment. My question is basically do I consider the axial load due to the floors and roof IF I have already considered these loads by calculating Ma as Zhuge has suggested in Step 2 of the last post.

I feel that allowing the floor and roof loads to cause moment on the wall, and then considering them again in the calculation of the deflected wall shape doubles the effect of these loads.

Thanks again,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

I guess I don't see why you are thinking you are counting loads twice.

Axial loads come in from gravity, they have eccentricity and cause moment, you get a deflected shape and the moment gets magnified a bit due to that deflected shape. The axial load is still just the axial load. The moment is 2% to 20% higher than the first order moment, depending on flexibility. Where are loads counted twice?

 

[Zhuge]

agree with JAE, you won't calculate them twice, because in Ma part, you only multiply gravity load with the eccentrity, and the result wont be as big as design moment.