Minimum Base Shear requirements for Dynamic Analysis

[rscassar]

ASCE states that a dynamic analysis must be limited to 85% of the base shear obtained from an equivalent static analysis. This provision isn't written explicitly in AS1170.4, but ethically is this limit being applied by engineers?

 

[Agent666]

There's certainly quite a few references to designing to NZS1170.5 when you have higher ductilities, design in higher seismic regions, etc. And NZS1170.5 certainly contains the same/similar provisions for base shear scaling (the min is 80%). So while not directly in 1170.4, it does reference the New Zealand standard for compliance in certain circumstances. So I'd say you could apply similar logic, but someone with more local Australian experience might be able to comment further on the local applications of AS1170.4.

Being from New Zealand myself, I can say NZS1170.5 is a far more complete standard in terms of addressing the design for earthquakes than compared to the what seems like a bit of a half hearted effort in AS1170.4.

 

[blihpandgeorge]

as1170.4 used to have a minimum scaling, but it has been removed. the preface to the previous amendment confirms this with "Scaling of results has been removed from the dynamic analysis."

 

[mrlm]

I believe AS 1170.4 (R2018) states that the base shear from a rigorous structural analysis should not be less than 70% of the value obtained from using the code period (i.e. static analysis). Refer to clause 6.2.3
As far as I am aware, this clause has not been removed.

 

[blihpandgeorge]

hi mrlm

the clause you reference is for static analysis only, note that it refers to period T1. This comes into play if you were to use a 'rigourous' analysis to determine the period but use that period for static analysis - then the static base shear (using a rigouous period) needs to be limited to 70% of the static shear with code period.

for a dynamic analysis in section 7, the period is defined as T (not T1 as per 6.2.3) in 7.2 however all other parts of the formula come from section 6

i too am surprised there is no lower limit, but there doesnt appear to be one

 

[mrlm]

That initially wasn't how I would have read that clause but would agree with what you have said.

See below clause C7.4.2.4(a) from the 1993 commentary to AS 1170.4
"The Clause requires the base shear force resulting from a dynamic analysis be increased to that required by Section 6. All corresponding forces should also comply with this increase. For the purpose of evaluating this base shear force increase factor, the period of the first mode may be used in calculating the earthquake design coefficient (C)by Clause 6.2.3. The value of C should be consistent with the calculated period but should not be less than 80% of the value obtained by use of the period calculated from Equation 6.2.3.
Dynamic analysis procedures are considered to provide a somewhat improved or more accurate force distribution. Therefore a base shear force of 90% of that determined by static analysis procedure of Section 6 is permitted for a regular structure when a dynamic analysis is performed. Also, when dynamic analysis is used, the fundamental period is provided and qualifies for the advantage given in Clause 6.2.4, where the resulting earthquake design coefficient (C) can be as low as 80% of the value determined using T calculated from Equations 6.2.4(1) and 6.2.4(2). Thus, it is possible to qualify for both directions. However, in order to prevent excessively low base shear force resulting from combining both reductions, the lower limit of 80% of the value obtained using T calculated from Equations 6.2.4(1) and 6.2.4(2) was set.
It is recommended that building structures classified as being irregular be designed for 100% of the base shear force value determined by static analysis of Section 6."

It seems the lower limit should be about 80% based on this information from the code and what others have said above.

 

[Trenno]

Only thing I can add is that Eurocode 8 doesn't require engineers to scale RSA base shears to a certain percentage of ESA base shears.

From what I understood, the reason for scaling RSA to ESA is because it was census that ESA would be conservative and there is more uncertainty associated with dynamic analysis.

 

[QSIIN]

My approach is to build a thorough 3D model and use the T1 period from the modal analysis in place of the T1 period from the code calculation, but only to the point that the base shear is reduced to 70% of that with the code T1, as per section 6.

But if you did a response spectrum, the code is silent on that one. It used to have explicit requirements in the previous edition of the seismic code, can't remember what year.

I have seen engineers incorrectly perform response spectrum, significantly underestimating a buildings stiffness, so that the building is flexible enough to reduce the base shear by well more than 50%.

And then wind governs yay no EQ detailing thanks

It runs rampant in Aus.

 

[li0ngalahad]

Firstly, we must underline that the Commitee have esplicitly removed the base shear scaling requirement for dynamic analysis when they have released the 2004 revision of the code. So it's not something merely missing, they actually decided to remove it. I believe the reason for it is to not constrain the designer to stick to an approximate formula that in many instances can be overly conservative. However, engineering judjement must always be used, and we cannot arbitrarly reduce earthquake load as much as we want by simply reducing the model stiffness. Models are too dependent on modificaiton factors (as well as other parameters such as releases or restraints) that are arbitrarly assigned.

So in my opinion the best approaches are two:
1) Stick to the scaling factor of 70 or 80% of base shear calculated from the approximate period formula;
2) Build a model with no releases and no modification factors or releases, run a dynamic RS analysis and use that as a minimum base shear to be used when subsequently changing the structure and introducing releases and modifiction factors.

The second approach is, in my opinion, the more accurate and the one reflecting the spirit of the code better. No scaling to a minimum shear based on an empirical approximate formula, but also engineering judgement tells us that we should not reduce the load simply based on arbitrary stiffness redution factors.

There is a third way of course, which would be to perform a full cracking assessment on every element of the structure and find the real cracked stiffness of the structure, however this would be quite complex and time consuming and I doubt any engineering company would adopt this for any standard project.