Axial Load Ratio Under Seismic 1

[ACS_1]

I have a question in regards to Axial load limit for vertical elements. It is for the latest Australian Standard but engineers outside Australia are welcomed to share your thoughts on this too.
Below is the clause.

So basically when ductility is assumed, the axial load ratio should be limited to 0.2. It says structural walls but I think it applies to columns too. Now my question is, does this still apply to columns/walls on basement when the structural base is assumed to be on ground floor? My understanding is this ratio is to ensure the displacement capacity of columns/walls so it should not apply to the columns/walls in the basement as displacement won't be an issue for them. What do you think?

 

[Agent666]

Same section, same reinforcement, same concrete strength. All things being equal except for axial load:-

Once above the axial load corresponding to the balance point, the concrete starts to control/affect the ductility/curvature that can be achieved within a cross section. A section will have less ductility (lower maximum curvature achieved) the higher the axial load once you are above the balance point.

 

[r13]

Same section, same reinforcement, same concrete strength. All things being equal except for axial load:

I think that's the point cristiano wanted to convey, but didn't spell it out the exact way as the above.

 

[IDS]

A common theme that seems to come through here on Engtips from Australian designers is one of how do we avoid these new provisions because they are so onerous, change the way we designed in the past, etc, etc. We get that here to, but we know the real consequences and loss of life that can occur when designers take shortcuts like this and argue semantics around code provisions and the like just to avoid doing the obvious.

I think this is an important point. Because earthquakes are rare in Australia, many engineers think there is no need to design for the extreme level event, but in fact the opposite is true.

In regions of high seismicity a structure that complies with the design and detailing rules for an "Ultimate Limit State"(ULS) earthquake will probably survive the greater loads of a "Maximum Considered Event"(MCE) earthquake without collapse. In regions of lower seismicity the ULS earthquake is much smaller, so that even if a structure fully complies with the requirements for that level, it will probably collapse under the MCE earthquake, because for these very rare events, although a low seismicity region MCE is smaller than a high seismicity MCE, it is still a very big earthquake.

The paradox is that although it is widely recognised that the increase in earthquake loads between ULS and MCE is greater in low seismic regions than it is in high seismic regions, design codes for high seismic regions specifically require that the MCE should be considered, but those in low seismic regions don't.

The changes in the latest version of AS 3600 go some way to addressing this problem by limiting loads on elements with inherently low ductility. This will not only provide better performance under very rare earthquake loading, but also under all other conditions where good ductility and robust performance are required, and those conditions are comparatively common.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[ACS_1]

I had actually worked in Asia, where seismic design played an important part in our design, for a few years before I moved to Australia. Things like strong column weak beam, stiffness ratio, displacement ratio, soft story, first mode etc in addition to axial load limit are what we would look at. I remember we had lower seismic requirement for level below basement one when the structural base is assumed to be on ground floor. I am not trying to avoid these new provisions. This thread is not to get rid of the axial load limit for ALL elements but for walls below structural base in particular. I don’t agree with this. Displacement capacity is the reason for axial load limit. If a column or a wall has no drift we can simply design it using iteration diagram without additional axial load limit (we already have phi to limit the axial load in iteration diagram design) This extra axial load limit is purely for earthquake due to its low drift capacity when heavily loaded, so I have no problem for this consideration for vertical elements above structural base. But for the basements that are lower than the structural base, with basements walls and soil all around, the drift of the diaphragm should not be an issue, which is why structural base is assumed to be on ground floor here to begin with, so why is the drift capacity of the columns/walls that connect to it is a concern here?
Also, AS3600-2018 doesn't mention this 20% axial load limit can be waived when you provide confinement as columns. (It doesn't mention this axial load limit should apply to columns as well, which is confusing). So I have to either increase the size or the strength to pass this 0.2 ratio limit to ensure the drift capacity of the elements that drift is actually not a concern? That doesn't make much sense to me.

AS3600-2018 has something similar but it is actual not “axial load limit” but more of when axial loads is over 0.3fcAg walls have to be designed as columns with confinement details. The axial load limit here is for the seismic weight and again, it doesn't say it can be waived by providing confinement.
And as I can recall, we had performance based seismic design when inelastic was required so instead of performing this 'modifying stiffness' simplified method we rather perform non-linear analysis when needed. So this global and local ductility is new to me. Can you walk me through how you adopt different ductility for different elements to New Zealand Standard? For example, if you assume the whole structure ductility to be 2 but you want to see if some walls can be designed as non-ductile, what do you do? What is the procedure be like?

 

[ACS_1]

Can columns be designed with P greater than the P at balance point? I think all standards allow you to do that as long as P-M falls in iteration diagram. The problem here is this extra axial load limit is for the seismic consideration, when the drift of the columns/walls are not an issue (I am not saying for all columns/walls), why do we still need to limit the axial load to this low?

 

[r13]

ACS,

The P-M diagram is constructed for an unique cross section with fixed dimensions (b & h), fc' and fy. There is only one balanced point. For seismic concern, the preference is design the column with the intercept point of axial force and the corresponding moment capacity falls within the tension failure zone, that is, falls within the area below the line drawn from the origin to the balanced point. However, for drift concern, the reverse is true, as the preference is the P-M intercepting point shall fall above the balanced point/line, which usually indicates a more rigid behavior when stressed to extreme.

 

[ACS_1]

. I get that it is better to design the column that way but even if your P falls above 0.3fcAg line it is still acceptable by the code as long as you provide proper confinement. And the drift capacity is the reason we want the column to have lower axial loads than P at balance point. The problem here is for the walls or columns under the structural base in particular, where the drift is very limited, applying the same axial load limit to those elements doesn't make much sense to me. If you look at the very first pic that I post, the condition here is mu>1, ie. if mu=1, this axial load limit can be waived. So the question here is, how will changing the mu affect the discontinued vertical element below the structural base? You reduce mu to 1 so the seismic input increase, ie the lateral loads applied above the structural base increase, and how will it affect the members below the base?

 

[r13]

ACS,

I understand where your thought came from - the basement is restrained by soil, thus displacement is limited (doomed to less than the base above). It is correct for most cases but earthquake, during which the soil and structure do not act in the same frequency, so the restraint might not be there through out the event. Also, somebody has pointed out already, the basement structure and the upper structure are continuation of each other, so if you have a ductility index of 1 for the upper structure, why the basement should be different? I don't think any code address this issue in a clear way, unless a complete/meaning soil-structure interaction analysis is performed.

 

[ACS_1]

If so, how can your ground floor assumed to be structural base in the first place? I am not talking about the continuous shear walls from above as I mentioned earlier. And I didn't say that mu = 1 for above structure and different mu for basement, what I wanted to say wss, what the difference will be for basement columns/walls that are not continuous from upper structure if we choose the mu=1 for the whole structure and re-analysis considering the base is assumed to be on ground floor? I don't think it will affect them because again, the structural base is on the ground floor.
When I worked for the company in HK we did design in China and I remember according to the Chinese Standard, columns/walls below basement one (if base is ground floor) can have lower seismic requirement, meaning the axial load limit will be increase as well. I am not sure if other codes have something similar?

 

[r13]

If so, how can your ground floor assumed to be structural base in the first place?

Because it is clearly indicated by the code, and all seismic design researches.

I am not talking about the continuous shear walls from above as I mentioned earlier. And I didn't say that mu = 1 for above structure and different mu for basement, what I wanted to say wss, what the difference will be for basement columns/walls that are not continuous from upper structure if we choose the mu=1 for the whole structure and re-analysis considering the base is assumed to be on ground floor? I don't think it will affect them because again, the structural base is on the ground floor.

Sorry, I don't understand what you are saying.

When I worked for the company in HK we did design in China and I remember according to the Chinese Standard, columns/walls below basement one (if base is ground floor) can have lower seismic requirement, meaning the axial load limit will be increase as well. I am not sure if other codes have something similar?

Don't meant to looking down on China, but do you remember it had quite a few deadly earthquake events in the past decades. I was in China for a project in the late 90s, good hard working engineers used out date codes, and work under real bad bureaucratic system. Good luck if you chose to follow Chinese code (could have been modernized) without check into other major code and researches.