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Eccentricity in Wind Load

Eccentricity in Wind Load

Eccentricity in Wind Load

(OP)
Hello guys,



ASCE7-10

In section 27.4.8
For the four load cases , torsion is considered to be 0.15 length of dimension in x,y direction and in section 27.4.6 another equation jump to calculate the eccentricity for non-rigid buildings, so my question is what exactly is the 0.15 means ,is this an accidental torsion and then I have to calculate the other torsion between center of geometry and center of rigidity and added to the 0.15 or it just included in the 0.15(based on experiment or code assumptions) , also in section 27.4.6 there is another equation for eccentricity for flexible building(so when I have to use it), so can any one summaries this miss

Also, why in totally flexible building, torsion is neglected? As in appendix D page 642

Generally I need Explanation for section 27.4.8, 27.4.6 and appendix D page 642 in ASCE7-10
Please make your explanation simple and explain the basics

Regards

RE: Eccentricity in Wind Load

(OP)
After Deep digging I found the following
:
26.4.8 eccentricity is for rigid diaphragm,and 26.4.6 equation is for flexible diaphragm,and the 0.15 eccentricity given by the table is to account for the difference between center of pressure(center of mass, and center of rigidity without the need for manual calculation(maybe based on practice)

Appendix D allow you to neglect torsion if you have regular torsional building which mean center of pressure almost with center of rigidity.


If all of the above is right

then

for my question

what is the different between flexible and rigid diaphragm and why in the flexible one we always neglect torsion+ section 26.4.6, does the eccentricity equation give more or less than rigid situation(it should be less)

Thanks

RE: Eccentricity in Wind Load

A Rigid Diaphragm, such as concrete slab, with an eccentric load, will rotate and maintain its square shape.

A Flexible Diaphragm, such as a roof deck, with an eccentric load, will change shape (parallelogram), so it will not rotate.

RE: Eccentricity in Wind Load

My state is still on ASCE-7-05 oddly enough. But I assume it hasn't changed much. My understanding is that it is indeed accidental Torsion because the wind may blow from an angle and you don't necessarily get a head on wind.

With that said, I take ALL of the ASCE-7 loads with a large grain of salt and employ a lot of judgment. People may say I'm wrong, and argue against me, but I have yet to design a building that looks anything like the nice square buildings they choose for the code examples. Therefore, employing the letter of the law to me is often impossible, at least in the sense that there is One way to do it. Therefore, I try to use judgment and think about how everything will interact together. People will disagree with me on this site, but I don't see too many threads on this site that all PE's are in agreement. Just remember the title of ASCE 7, MINIMUM loads. And use your best judgment based on what the code is pushing you to. Whether that's figuring out the exact eccentricity, or just increasing your loads by a reasonable factor to account for the fact that the loads could be off center.

Point is, we aren't designing swiss watches here. And the wind doesn't care what the code says, nor do earthquakes. Unless you have a building that looks exactly like the code pictures, I do my best to follow the intent of the code and move on and don't sweat it too much. That, and I don't take things to 100% of capacity.

One other thing before I end my rant...Remember that the code is there to cover almost any building you can dream up. I feel like a lot of the code is there for high-rise structures where redundancy, and regularity play a much bigger factor. I think a lot of structural engineers that do small 1-3 story structures are stuck using a complex code that doesn't always seem to apply easily. What I mean is that the intent of the code applies, but the reality isn't there. For example, find me an engineer that can design a 1 story typical load bearing masonry structure for a high school where an architect has all kinds of crazy features. I think if you put 10 engineers in a room, you'd have 10 different designs and they would all argue that the other engineers had "code violations" in them.

Lastly, I think that these complex things that the code folks dream up actually can have the effect of reducing life safety. All of a sudden the engineer has to be removed more from the design process to rely on a computer with thousands of inputs. I can do all 100+ load combinations on RISA and then I just do a 2-minute sanity hand calculation and I'm usually within 10%. But, without doing the hand check I could easily make an error and not know it because there are so many calculations behind the scene. Every calculation by RISA is engineering judgment delegated to the computer. That's not necessarily bad, but can be if the code continues to get so complex that we can't do a quick rough hand calc to verify things. Then, it's all relying on the computer. I trust the computer but I've found errors in the RISA program myself, not to mention the countless input errors I have made that I of course go back and correct. And it was quick hand calcs that allowed me to catch them.

/rant. And maybe I should start a new thread. I don't want this want to shift gears..Just food for your thoughts as you try to make sense of ASCE7

RE: Eccentricity in Wind Load

(OP)
Thank you all

really helpful

njlutzwe

So YOU are saying that 0.15 is accidental only, but according to my understanding to the code it is accidental + e which comes from difference of center of pressure and center of rigidity.

And I would like to say that I totally agree with what you said regarding the code.


RE: Eccentricity in Wind Load

drasticxxxx,

I think it's just accidental (may not be the right word, but point remains). I could be looking at this wrong, but I think that this "wind" eccentricity is related to "applied" loads. The center of rigidity is related to the "resisting" forces, IE structural response to the wind loads. If you have a structure with an offset center of rigidity, my understanding of the code is that you would apply the wind loads (including the moment) from Figure 6-9 (I'm using ASCE7-2005) to the structure. Then, as you analyze the rigid diaphragm (or let your software help you), it will distribute the loads to the structure and that torsional effect from the offset C.O.R. will be taken into account in the same way that you would expect any other loads to be taken into account. To me, this "wind" eccentricity is just on the applied load side.

Reason being is that the ex and ey as defined in figure 6-9 are only related to the plan dimensions, with no care where the C.O.R. actually lies.

That's my interpretation anyway.

RE: Eccentricity in Wind Load

Wind pressures can be greater on the left side the building and lesser on the right. Wind is not uniform. This phenomena has a greater impact on rigid diaphragms, which twist.

I'm with njlutzwe in that you should not get hung up on it. The code is describing the behavior, and stating that it exists. Be aware of it.

RE: Eccentricity in Wind Load

(OP)
Thanks again guys

RE: Eccentricity in Wind Load

It is 'accidental' plus/minus actual. Look at the 'simplified' method and you will see higher pressures some distance in from one edge. This is that same 'accidental' eccentricity.

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