AS 1170.4 Clause 1.1 - Building with a Period Greater than 5s 1

[tmac7285]

The 1170.4 clause 1.1 (g) states that buildings with a first mode period greater than 5 seconds is outside the scope of the standard. Where or whom are we supposed to go to for this approach?

1170.2 has a similar criterion and that the code based wind does not include any buildings with a period greater than 5 seconds, amongst other criteria. It's clear that we should consult a wind engineer and get the building tested in a wind tunnel. This is not so clear for earthquake.

Is the aforementioned 1170.4 clause essentially not representative of the taller structures we are building now? Can this clause be ignored if we undertake a rigorous analysis?

 

[Agent666]

For comparison in NZS1170.5, there is no similar period upper limit.

But this is perhaps addressed in part by two things that AS1170.4 does not specifically address in my opinion: -

1. the fact that in NZS1170.5 there are minimum limits for the seismic coefficient providing a minimum level of base shear irrespective of period. As far as I can tell nothing similar exists in AS1170.4. This limit starts to kick in below a period of 5 seconds depending on ductility.
2. the fact that NZS1170.5 has a minimum cut-off for Z of 0.10, even when certain parts of the country lie below this level. As far as I can tell you have no such cut-off in AS1170.4.

In other words, some minimum level of seismic risk must be considered irrespective of the period. How would you determine this for a period of greater than 5 seconds, this is getting into the realm of site-specific special studies by geophysicists to determine a site-specific design spectrum that maintains a constant level of the risk.

What is your definition of a rigorous analysis you mentioned? Doing a more rigorous analysis does not inform in relation to seismic risk which is what I believe they are angling for in terms of limiting the applicability of the standard to 5 seconds or less.

https://engineervsheep.com

 

[tmac7285]

Thanks Agent666.

1. There is a minimum base shear when considering using etabs periods. The minimum is 70% of the shear that the code produces. However, I believe the clause relating to minimum base shear for a modal analysis has been subsequently removed, though I think there are at least some engineers that also apply a minimum 70% base shear for code generated forces.
2. There is also a minimum Z of sorts. This was introduced relatively recently in an amendment to the code and is kp x Z=0.08 I believe for a 1/500 earthquake

I've never heard of anyone conducting a site specific "special" study carried out by an appropriately qualified party, though I'm not experienced enough to say it's never happened; it certainly isn't industry practice. I figured that may be the answer, but equally the clause may be a hangover from the days when having tall buildings were rare. There must be a bunch of engineers ignoring this clause, because having a building period over 5 seconds is not that rare.

 

[rowingengineer]

I think you will find that a number of consultants use performance solutions to meet the requirements, I know for a few specialised structures aka wind turbines, they referenced appropriate international standards and one of them was nzs1170.5.

 

[Agent666]

Regarding point 1, that isn't what I was referring to in particular. In NZS1170.5 there is a minimum limit of 0.03Ru or (Z/20 + 0.02)Ru and similar for SLS1 and SLS2. So basically after a given period your seismic coefficient is constant.

I've been involved in one recent project where the client did a site specific study. It showed quite a different spectrum than the average design 1170 spectrum. At lower periods it exceeded the standard design spectrum, and at higher periods a lower design load could be utilised. But really for an informed client it was about ensuring their seismic risk was addressed, for example if code values were used may not be achieving code intent for the lower period structures on the site. It wasn't really about demonstrating that a lower load can be utilised, but often you will end up with this occuring over some range of the resulting site specific spectrum.

Never seen or dealt with a structure that has a 5 second or greater period at the ULS. Struggling to imagine what this might even look like.

https://engineervsheep.com

 

[mrlm]

For a building to have a period of >5s it would have to be in the order of 200m tall or 50 storeys (using the old approximations). Surely there would be issues with accelerations exceeding occupancy comfort in the structure?

 

[Retrograde]

For a building to have a period of >5s it would have to be in the order of 200m tall or 50 storeys

The design of a building that tall in Australia will be dominated by wind load considerations.

 

[tmac7285]

mrlm, as Retrograde suggests these are typically wind dominated buildings (not that we can ignore EQ, particularly with the new code). There have been a couple of buildings pass through our office in the 130m-160m range that have stability systems that are less stiff than what the typical rules of thumb would suggest and tend to working hard. They are subject to wind tunnel testing, which includes acceleration checks. You're right in that some have been right on the line for what is recommended for accelerations, but typically haven't required any damper tanks, though some have on projects I'm not involved in.

Agent666, I get what you're saying. Was the site specific investigation an expensive item? Sounds like it would be an interesting report to read. It would be pretty hard to convince anyone in Australia to carry out something like this, given it's hard enough to convince us engineers that EQ is real here and needs to be properly considered. I don't like ignoring clauses, but perhaps I could put a minimum seismic coefficient similar to NZ. It's not like etabs says "oh no, over the 5 secs, can't analyse", it still provides answers, so I might check how reasonable these are in light of your comments.

 

[blihpandgeorge]

i have had a site specific seismic hazard assessment (SSSHA) done on a project and got the same results as Agent666 noted (for some return periods), higher loads at low periods and vice versa at high periods. As most structures sat in the lower period it was a decent increase over code. interestingly enough, the increase was only on some return periods, whereas other return periods were lower than code.

agree with retrograde that a tall building in Aus will be likely governed by wind, from my experience anything in concrete less than 9-10 storeys is seismically governed (if located in one of the Z=0.08 locations). And above this is wind governed. You still need to check seismic drifts and detail accordingly, it is just that seismic tends not to be the governing criteria for primary structure. It still govern for parts and non-loadbearing elements though.

 

[Agent666]

Was the site specific investigation an expensive item

I was party to how much it cost, but even if 50k-100k for example, even a small saving in structure due to improved loadings would easily offset the cost. If you're building a 100 storey building it should be a no brainer to do it.

Maybe even insurance risk being more informed, saves in insurance premiums in the longer term to offset initial costs of the study.

https://engineervsheep.com

 

[saplanti]

I would suggest you to ask your client if they have site specific response spectra for that particular site. Using the spectra will solve many problems.
Most of NZ Clients are aware of the site specific response spectra, or they may divert you to a consultancy firm to produce one or provide it if available.

 

[tmac7285]

saplanti, I'm in Australia and what you are suggesting is just not common practice and no client here would know what response spectra is, let alone have that kind of specific site information. EQ rarely governs the design for taller buildings here, due to the lower seismicity acknowledged in the code. It is becoming a more important consideration with the code updates, but we are still pretty unsophisticated with regards to EQ.
Agent666, I don't believe any savings would be ultimately realised, given that the dominant actions are wind. There's more to my previous point in regards to detailing for the assumed ductility etc, but in essence, the tension reinforcement in the stability elements and header beam reinforcement will be sized based on the wind loads, given that an assumed ductility Mu=2 and Sp=0.77 yields lower forces than the wind case (even if we used code based equivalent static forces). It would be a pretty hard sell to the client to commission a site investigation of the specific response specta as an extra expense, for nothing gained (except compliance with a small code sub-clause).

 

[saplanti]

tmac7285,

The period limitation of using the code rules was the concern. This is why I referred the site specific response spectra. I know that Australian Seismic Code does not show you large seismic shear load, however I would try one of the code response spectra which will represent the site for the high rise building. If you are using SpaceGass or Microstrand there are some embedded specific spectras for several locations in Australia. One of them may be applicable or close to see the seismic implementation on the structure.

I understand there are differences between NZ and Australian earthquakes, and wind has the governing shear force. When you implement one of the spectra in the software that some level’s shear forces become more than wind shear forces for those elevations. This is not surprise. Therefore, it is worth to see in the computer applications.

 

[Agent666]

Tnac, well your seismic loads will use up the exact capacity you provide for wind to obtain your ductile mechanism.

So unless the wind forces are greater than the MCE level event loads or ductility 1 loads I hate to break it to you but wind won't necessarily govern.

Sooner or later there is potential for seismic loads to be equal to the wind if you're designing for some moderate level of ductility. What you should really be comparing wind vs ductility 1 loads to determine if wind actually governs vs seismic. Otherwise if you design for wind and ductility of 1 loads are higher, then both wind and seismic will govern. Your argument of comparing a higher ductility loads to wind is flawed, you have to think more in terms of the actual ductility your strength and detailing provided will actually attain in practice when forming your chosen ductile mechanism.

In NZ where we have real seismic loads and wind is cyclonic savings are certainly possible.

If your clients are uneducated regarding seismic, consider educating them.

https://engineervsheep.com

 

[JohnWk]

For The 1170.4 clause 1.1 . I would still use the standard as higher period will cause less Earthquake force , so use 5s rather than a higher value is conservative.

For 1170.2. that 5sec limits is for wind serviceability and human comfort conditions , you should ask for wind tunnel study but first revisit the analysis model which should be different than the one used for Earthquake such as.
[ul]
[li]restrain supports rather than use springs[/li]
[li]use 1 year elastic modulus[/li]
[li]use dynamic Elastic modulus which is about 20% more than elastic, this is to get the acceleration from code or to wind tunnel consultant[/li]
[li]use service condition stiffness reduction (uncracked sections)[/li]
[/ul]
I did many high rise projects and you will not get anything more than 5s for 200m or less building unless you have pretty bad structural configuration/core size
if the structure is over 200m you need to get wind tunnel done anyway as cross wind and shielding factors will be different from the code