Wind loads - servicaebility and ultimate 4

[ndrstruct]

Hi, I am new to this forum and I am new to Australian standards. For wind load calculations, I have seen serviceability and ultimate wind speed. I am wondering why there are two categories for wind. Where to use this serviceability wind loads?

Thankyou

 

[271828]

I'm not familiar with Australian standards, but I can give some background on the ASCE methods for this. Seems like the philosophy would be similar.

The main body of ASCE 7 has wind speeds that correspond to various very long mean recurrence intervals. These are used for strength and stability -- safety -- checks. That's probably what is meant by "ultimate" in your post.

Appendix CC, Figure CC.2-2, has 10-year mean recurrent interval wind speeds that are much lower than the ones mentioned above. These can be used for wind drift serviceability. Because this isn't about life safety, the loads can be reduced. Also, I'm pretty sure this part of ASCE 7 isn't mandatory whereas the ones mentioned above are.

 

[phamENG]

Also only work with ASCE, but I'll take a stab it in slightly different wording:

Serviceability is about comfort and maintenance. How much does the building sway? Can occupants notice it? Does it crack the plaster?

Ultimate is about making sure occupants don't die. It that kind of a storm (which is unlikely to happen during the design life of the building, but the chances are just high enough that we worry about it), we don't care if the occupants are comfortable or if plaster cracks so long as they survive it. You do have to consider sway since it can have secondary effects on the strength of the structure, but the limits are based on strength and not on drift for finish damage or occupant comfort.

Making occupants comfortable in a 700+ year MRI storm would be very expensive.

 

[human909]

A reasonable question. If you haven't already, read 1170.0 1170.1 and 1170.2, but I presume you've gotten that far.

Serviceability wind loads are required for building serviceability conditions which might include occupancy comfort, integrity of non structural items such as partitions or operational requirements for machinery like gantry cranes in a portal fram building.

In my engineering career of industrial structures I've rarely used serviceability wind loads except for basic portal frame shed buildings.

Generally with braced frame structures you end up with a structure that is stiff enough if you design for strength. But structures that are less stuff serviceability wind loads come into play.

In Australia serviceability limits are generally guidelines rather than strictly required. That said, the guidelines given are not onerous and often are unconservative for discerning clients. So if unsure treat the guidelines as bare minimums and consider applying stricter deflection criteria if the client is after a well performing structure.

 

[Tomfh]

I practice in Australia. Ultimate wind speed is the wind load the structure needs to be able to resist without failing, e.g. members breaking, or the structure blowing over. Serviceabilty wind speed is less critical and is more about keeping people comfortable and maintaining the structures. E.g. A roof or wall shouldn't deflect/bend too much when the wind blows. A window support beams shouldn't deflect so much that the glass breaks.

Servicability wind loads often govern for commercial/residential/educational structures. What serviceability limits to apply is a bit of a grey area.

 

[ndrstruct]

Hi guys, I appreciate all responses and your valuable time. I was practicing with US standards (ASCE) and in there we have a wind speed (No separate category for ultimate and service) and an appropriate load factor for strength design.
When I started working with Australian standards I have seen a separate speed to strength design. In AS 4055,I saw simplified wind pressure calculated and when referring to pressure in cyclonic regions, I noticed for ultimate (+5.33 kPa) and for service (+1.63kPa). its almost (X 3.2 times higher) and it made me confused.
I believe structures designed with this much ultimate pressures would be stiff enough for deflections and additional service checks for these small value may not have to be performed.

Thankyou

 

[human909]

I believe structures designed with this much ultimate pressures would be stiff enough for deflections and additional service checks for these small value may not have to be performed.

Yes cyclonic winds are a whole 'nuther beast! AS4055 is the baby document for the general 1170.2.

As already discussed whether serviceability or strength is the determining factor usually depends on stiffness of the structure (braced/moment frame, aspect ratio), the role of the structure, and also as you have found for wind if it is in a cyclonic region or not.

See below for the key documents. 1170.0 Covers combinations and has servicability guidelines. 1170.2 is where the real fun starts with wind loads!

https://files.engineering.com/getfi...c-1ff8-4f0b-9747-0504611a7ef6&file=1170.0.pdf

https://files.engineering.com/getfi...9-77e6-4dad-8a8d-aeed0762da64&file=1170.2.pdf

 

[Tomfh]

I noticed for ultimate (+5.33 kPa) and for service (+1.63kPa)

Which cases are these? It's uncommon for ultimate to be that much higher than service wind.

I believe structures designed with this much ultimate pressures would be stiff enough for deflections and additional service checks for these small value may not have to be performed.

Maybe, but not necessarily.

 

[Retrograde]

Which cases are these? It's uncommon for ultimate to be that much higher than service wind.

For wind class C4 these are the values for walls (any position) from Table 3.3 and 3.4 of AS4055.

 

[phamENG]

Just because the building is strong enough to resist the ultimate load doesn't mean it won't sway like an amusement ride in service level winds. As human mentioned, some building types are inherently rigid, others are not. Concrete shear walls? Probably don't need to worry about serviceability too much. 10 story steel moment frame? You bet you need to consider it.

 

[ThomasH]

I noticed for ultimate (+5.33 kPa) and for service (+1.63kPa). its almost (X 3.2 times higher) and it made me confused.

If you take into consideration that the pressure from the windload is proportional to the wind velocity squared, it may be less dramatic. What was the wind velocities for the pressures you have?

In my experience, for wind, the requirements for ULS and SLS can be very different. If you are working with a higher building dynamics come into play and SLS requirements can be acceleration limits. I would say that considering only strength for a higher building is not enough.

 

[ElliottJames]

Wow. I have a different view of what Ultimate wind loads are. It has nothing to do with safety. Using 1.2D + 1.0W + L combo for example, note the lack of load factors except 1.2 for dead load. When designing to this load combo, the structure is at the yield point. It is on the knife's edge of complete failure and it is not a safe place to be. (Only talking about wind here, and am excluding designing for ductility to dissipate seismic energy).

In ASCE 7-05 load factors acted as safety factors, such as 1.2D + 1.6W, on 50- and 100-yr MRIs. At those wind speeds you could say the structures are completely safe because the 1.6 factor gave some buffer before yield is reached. What the code committee did was remove load factors and instead went with higher MRIs of 300 years (class I) or 700 years (class II). In essence they moved the safety factors from the calculated loads into the higher wind speeds that an MRI of 300 yrs/700 yrs represent. Personally I think a more honest approach with the public is to say, "We expect the structure to begin to fail and accrue damage at around 130 mph, but that wind speed is very rare and is only expected to occur once every 700 years."

 

[IDS]

Personally I think a more honest approach with the public is to say, "We expect the structure to begin to fail and accrue damage at around 130 mph, but that wind speed is very rare and is only expected to occur once every 700 years

In what way is it dishonest to replace an arbitrarily selected maximum wind speed combined with an arbitrarily selected "safety factor" with wind speeds based on long term records combined with a return period based on the consequences of failure of the structure, which in many cases results in a structure with greater wind resistance?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

You could just as easily factor the 50 year wind and get the same answer as an unfactored 500 year wind. I’m not sure how it’s dishonest. It’s simply the way the codes are set up that you get the ultimate wind load directly.

 

[rowingengineer]

As4055 service pressures are incorrect in my opinion. I have flagged this with the committee.

 

[human909]

For those reading along and are a little confused at the high variance between serviceability and ultimate it should be emphasised that these values are for a cyclonic zones so it is to be expected.

Eg;
At short recurrence periods the winds are barely higher than 'normal' regions. However as the recurrence period increase the wind load increases extremely rapidly. It is entirely sensible and rational to have a big difference here, if the once in a 500 year cyclone comes along you want your structure to stay intact, who cares about the serviceability. Most years are much more sedate and the strong storm is what you want you structure to stay serviceable in.​

Wow. I have a different view of what Ultimate wind loads are. It has nothing to do with safety. Using 1.2D + 1.0W + L combo for example, note the lack of load factors except 1.2 for dead load. When designing to this load combo, the structure is at the yield point. It is on the knife's edge of complete failure and it is not a safe place to be. (Only talking about wind here, and am excluding designing for ductility to dissipate seismic energy).

A couple of people have already pointed how this assertion incorrect about the safety.

In AS codes both wind and seismic are designed as genuine 'ULTIMATE' genuinely limit state. That is how the code calculates the seismic and wind loads. Personally I'm happy if my structure is calculated to withstand winds of once in 500 years. I already round up in my calcs and my member sizes so after all that it would probably be suitable for a one in 2000 year even which is only winds 10kph higher.

So yeah if a "knife's edge" event occurs less often than 1 in 2000 years then I think I'm sitting pretty comfortably.

 

[ThomasH]

If the mentioned load were for cyclonic zones my comment was probably not very relevant .

Wow. I have a different view of what Ultimate wind loads are. It has nothing to do with safety.

In my experience (primarily Eurocode) we start with a basic wind velocity (50 year return period) and calculate a wind load based on a number of parameters. Then we use load factors to create load combinations. And Ultimate Limit State (ULS) load combinations are about safety .

I have worked with other codes also and I don't think I have seen a code where ULS and ALS (Accidental) hasn't been about safety.