Are structures to remain elastic under wind loading? 16

[amengr]

Just a quick question as I am new to wind engineering. For the MWFRS are the wind loads we calculate per ASCE7 such that we are designing the structure to remain elastic or is it similar to seismic design where we allow structures to develop plastic hinges/deformation?

 

[dik]

@Pham: They can be, but I don't and also for seismic. You have to consider 'shakedown' analysis for loads that may repeat (and in a different direction, like wind, or seismic). The initial deformation, if taken into the plastic region, will leave residual stresses and the structure will behave elastically up to that loading if the load is re-applied. These residual stress can cause premature failure if another different load is applied and the residual stresses act in a manner to weaken the structure with the new loading.

I never consider columns for plastic design... I'll find my economies elsewhere... with columns and cantilevers, there is little redistribution.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ChorasDen]

KootK, to follow up with my agreement to you.

Can we try and least look at the code for some of this discussion?

Per the commentary to chapter 26 of ASCE 7-16:

I read this as indicating that ASCE 7-16 assumes elastic structural performance for wind design.

 

[dik]

I'm not sure this is a good example... to me the diaphragm generally takes the load to the structure... it says nothing about the design of the structure. Also for chimneys, because there is no redistribution, they should be elastic.

As I noted, for wind and earthquake, I don't use plastic design due to shakedown; if accounted for, I'm not aware aware of any limitations. For simple rigid frames, shakedown can be a real problem, and the problem gets worse for complicated structures.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

Can we try and least look at the code for some of this discussion?

In my opinion we should not pursue that. Building a plastic, whole frame mechanism into a wind resisting vertical frame doesn't violate code, it violates statics. It violates statics in the same way that a cantilever column with a true hinge at the base would violate statics:

1) A static, lateral load at the top would cause it to collapse.

2) If #1 were not enough, the axial load applied to the column would cause it to become unstable.

It's that simple. I can feel this thing going sideways so, please, everybody recognize that my opinion on this is informed, utterly, by this assumption:

I believe that what OP is asking is this: can/should a vertical, wind force resisting lateral system form a plastic frame mechanism as many seismic systems are designed to.

I've not been talking about:

1) A single member within a frame going plastic and redistributing its load to other parts of the frame OR;

2) One frame of many within a framing line going plastic and redistributing its load to other frame within the same line.

Those strategies would be highly unusual, but not unworkable.

What I've been talking about is the strategy that we use commonly in seismic design whereby a building's entire lateral system forms a plastic mechanism that results in that system ceasing to attract additional seismic load. And, again, that is only possible because:

Seismic systems can only go plastic because of the cyclical and inertial nature of the load.

If other people here are speaking to other things, then that is their prerogative. What I've just described is what I am speaking to and what I believe formed the crux of OP's question.

 

[dik]

When dealing with plastic design, you are dealing with mechanisms... and statics is not applicable. The reason it may stand up is that there may be strain hardening... If you can imagine a rigid frame, perfectly proportioned to resist moments... the entire frame would go into plastic mode at the design load, and would become a 'puddle'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

When dealing with plastic design, you are dealing with mechanisms... and statics is not applicable. The reason it may stand up is that there may be strain hardening... If you can imagine a rigid frame, perfectly proportioned to resist moments... the entire frame would go into plastic mode at the design load, and would become a 'puddle'.

In my opinion, that simply is not correct dik. Two reasons:

1) Plastic mechanisms under a static load generally have to satisfy static equilibrium and, therefore, statics.


2) A seismic frame gets its load reduced from the elastic value because it is assumed that it will plow right through initial plastic hinge development and strain hardening such that it will form true hinges that stop additional load from being attracted to the frame.

Yes, one could theoretically design a wind frame to go into the plastic range so long as it was designed such that strain hardening kept the frame from ever truly forming a mechanism. But, again, I believe that OP is enquiring about something much more pedestrian and straight forward than that.

 

[chris3eb]

Just a quick question as I am new to wind engineering. For the MWFRS are the wind loads we calculate per ASCE7 such that we are designing the structure to remain elastic or is it similar to seismic design where we allow structures to develop plastic hinges/deformation?

I think what OP is getting at is that the code-based seismic forces are specifically reduced (ASCE 7 R-factor) to account for the fact that the building will behave nonlinearly. This happens without the engineer choosing to consider plastic mechanisms.

I don't think the question is should you use plastic design principles for wind design. I think the question is whether the code based wind forces are based on plastic hinges being developed. And I think the answer to that question is no, they are based on the structure remaining elastic.

I think the reasoning why you wouldn't want to develop plastic mechanisms for wind is because it will cause permanent damage and it just isn't necessary to do that for wind events.

 

[KootK]

I think what OP is getting at is that the code-based seismic forces are specifically reduced (ASCE 7 R-factor) to account for the fact that the building will behave nonlinearly. This happens without the engineer choosing to consider plastic mechanisms.

I agree with there being non-linear behavior but disagree with the assertion that the designer need not consider plastic mechanismsat least for the capacity design approach which underpins our most common methods of seismic design. If a structure has it's seismic load reduced by an R-value but does not form a mechanism under the associated seismic event, I would say that those loads were erroneous.

 

[dik]

The following is my response to an earlier eMail on the subject:

Going plastic does not mean, nor denote, collapse.

This has changed over the years... Plastic design, today, means using the Plastic Section Modulus (Z) and I don't consider this as plastic design. In the good old days, plastic design meant determining the 'collapse mechanism', assigning the limiting moment to the plastic section capacity.

A simple beam that develops a plastic hinge does not continue to deflect and then collapse, even under the loads at which the hinge was developed. This is a very common misconception, and is wholly erroneous.

You can design a simple beam using Z, if it is a Class 1 or Class 2 section; this, in my opinion, is not plastic design. You are simply using the code prescribed section resistance. Deflections are just greater... once the hinge forms, if it weren't for strain hardening, it would continue to deflect... one hinge, one degree of freedom... a mechanism.

NOW: I am also a huge fan of Capacity Design, wherein we design a system to never allow collapse no matter the load. So, I am not saying that this wouldn't require attention to detail(s), but I see no reason to code-blanket-proscribe plastic in a wind frame.

You can use plastic design for wind loads with a caution. When a structure deforms into the plastic range and the load is released, if it is again taken to the original load, it will behave elastically. There are residual stresses that remain. The problem occurs if there is a different loading. The 'residual stresses' may function in a manner that will cause collapse. This is commonly referred to as 'shakedown'. If you can get alternating loads, this can become a concern. I don't normally use plastic design for wind loading, and I would definitely not use it for seismic loading.

The best book, explaining this IMHO, is an oldie, "Maissonette and Save (sp?) Plastic Design-Vol 1". I couldn't find it in a search... spelling may be wrong, but the text is excellent. It goes back 60 years.

NOW: I am also a huge fan of Capacity Design, wherein we design a system to never allow collapse no matter the load. So, I am not saying that this wouldn't require attention to detail(s), but I see no reason to code-blanket-proscribe plastic in a wind frame.


I've designed hundreds of structures using plastic design... and have no concerns. These are generally warehouse type structures where there is a large number of equal length spans. The attached Vista Cargo project was done using plastic design. Both buildings, and to give you an idea of the scale, those are tractor semi-trailers... It was the first plastic design structure that came out of [deleted]. Properly executed, there is a huge economy in using 'plastic design'. When I first started, a lot of 'wise old engineers' told me that there was a 25% premium for using it... myth... there are fewer sections to handle and lighter weight. I don't include columns in plastic design, pin connections only; I like redundancy, even if I don't use it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

Not a word of your last post applies here dik. You were speaking to systems which can go plastic locally and redistribute their loads to other places. A building's lateral system cannot form a plastic mechanism under wind load for the same reason that the column shown below cannot form a plastic mechanism at its base: there's nowhere to distribute the load to.

 

[dik]

thanks for the clarification... as I noted (a few times), I don't use plastic design for wind loading or seismic, and for the reasons stipulated... if I had a frame with a non-varying horizontal load, I might consider using it for that purpose.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[CELinOttawa]

I find it interesting that people seem to think that a plastic hinge should be allowed to form in a column. It should *not*. Never. Not in any form of design I am comfortable or familiar with!

I would happily have any mechanism that involves hinges in my beams. That's still plastic design. Hinges in a column? No. Not for wind, not for seismic, not for gravity.

I still haven't seen anything to dissuade me from my views; Plasticity in the beams is in no way dangerous, even for wind.

 

[WARose]

I think the reasoning why you wouldn't want to develop plastic mechanisms for wind is because it will cause permanent damage and it just isn't necessary to do that for wind events.

It's not just that....it's cycles to failure. There is no comparison between the expected number of cycles in a seismic event and wind event.

 

[dik]

I treat columns as pinned (except as part of a rigid frame, then I deal with them appropriately)... they are not part of my plastic design model...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[chris3eb]

@KootK - what you're saying is a little circular, at least when it comes to the LFRS. If the actual seismic loads don't cause any plastic hinges to form, then each element remains elastic (and not overstressed) for the actual seismic loads? So then why would looking at reduced seismic loads be a problem? The only thing would be if you couldn't develop a flexural hinge prior to shear failure or something similar, but as far as I know that is handled at prescriptively, so it's independent of your actual loading.

The reduced seismic demand (ie 1/R factor for ASCE 7) doesn't require a whole building mechanism (ie instability). Individual elements can go plastic at different times and reduce the seismic demand to the entire structure (vs if everything stayed linear). This is where the reduced demand comes from - not because a stress-strain curve of the building as a whole goes perfectly plastic.

Here are some pushover curves on a frame/building level - they begin to soften as plastic hinges form, but they never go completely flat or negative and if they did, that would be a problem.

 

[chris3eb]

@CELinOttowa there is no problem in hinges forming at the base of the column of a fixed-base rigid frame. It then becomes a pin-base rigid frame... Obviously for KootK's example of a cantilever column, forming a hinge is bad because it forms an instability, but that isn't universal

 

[WARose]

I would happily have any mechanism that involves hinges in my beams. That's still plastic design. Hinges in a column? No. Not for wind, not for seismic, not for gravity.

Don't do any push-over analysis then. Hinges in the columns aren't unusual there.

 

[CELinOttawa]

I'll happily limit myself to having the columns dance around hinges in the beams, thanks very much.

Lots of room to have energy dissipation without engaging risk by allowing the columns to go plastic.

 

[KootK]

I find it interesting that people seem to think that a plastic hinge should be allowed to form in a column. It should *not*. Never.

Even that isn't strictly correct.

1) Every capacity designed column that is part of the lateral system has to form a plastic hinge at it's base. Without that, the lateral system cannot be said to have formed a mechanism and, therefore, cannot be said to have stopped attracting lateral load.

2) It's fairly common to allow plastic hinges in columns at the roof level.

 

[CELinOttawa]

Wait a minute...

I think we are at a "terminology" crossroad again.

Would we all agree on "Strong column, weak beam" or / aka "Strong column, weak axis"?

PLAN for beams to form hinges. DETAIL for columns to allow plasticity and loss of concrete.

We design for the beams to go fully plastic, and we overstrength our columns to make them unlikely/nearly never the subject of energy dissipation.

If the earthquake keep a rockin', then the columns must start a knockin'..... But we don't WANT the columns to form hinges, so they are subject to overstrength.