# Seismic Weight and Tributary Wall Heights



## Casey

When calculating the wall weights to be used in for the seismic weight I see two approaches when dealing with the ground floor (or a single story building)

The first one is that the entire height of the wall is considered as part of the seismic weight. Williams likes to do this in his examples.

The second one is that only the upper half of the wall is considered as part of the seismic weight. Ref SEOC Seismic Design Manual Vol III Ex 1A page 11. The author applies the upper half of the ground floor wall to the second floor. Similarly for Ex 3 pg 127

Is using the entire wall just a conservative approach? Or is there a decent justification out there?

I personally agree with the second approach as I visualise the weight from the lower half of the wall being distributed to the foundation and not the SLRS. Can anyone show me why this approach is not appropriate?

As for parapets, I notice there is two trains of thought on how they along with the wall below the roof should be distributed to the roof diaphragm. One approach is a straight forward tributary calculation with exactly half the wall between the roof and floor below going to the roof diaphragm (the upper half obviously) plus the full height of the parapet. (SEAOC Seismic Design Manual Vol I pg 165-166 does this)

The other method ratios the total height with parapet over the height of the level below (basically treats as a simple beam with an overhang) [sEAOC Seismic Vol II Ex 5 pg 260]

Again, other than one method being slightly more conservative than the other I don't see a huge difference. Can this one be chalked down to design preference/laziness? Or is there a particular reason why I should follow one approach over the other?

Any input would be great.

Thanks

Casey


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## kevo_55

Casey,

The way it was explained to me was that it all depends if your walls are rigid or flexible.

If you have rigid walls, you look at the weight tributary to each level like a continious beam. (you are just solving for the support reactions)

In flexable walls, it is simply the tributary area to each level.

The better question is what is truely rigid and truely flexible? In most highrise buildings, cladding is designed as a rigid beam going up the entire building. I suppose in these situations I would treat the walls as rigid.

In lowrise buildings I would most likely treat the walls as flexible.

Just my :2cents:


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## MOOK

I had the same question in my mind long time ago and I raised this question but I did not get clear answer.

I do not think this is related to the type of the wall (flexible or rigid).

To determine the Seismic Force on Buildings:

For one story buildings; some reference used the whole weight of walls (without any information about its type) and other reference use half of the weight. The majority use half of the weight.

For two story or more, all reference use half the weight of the first story + half of the weight of the second story walls.

To study Shear Wall Stability:

In Allan William books, he used the whole weight of the shear wall (timber wall) to get the seismic force due to the wall selfweight to study shear wall stability. Other references used half of the wall weight (It is the same timber wall). Moreover, in the same Book of Allan Williams, he used only half of the wall weight to determine the seismic force on the building despite he used the whole wall weight to study the stability of the wall few pages earlier.

To determine the Diaphragm Forces:

All references agreed to use half of the wall weight.

My guess is there is no any theoretical bases to determine if you have to use half of the wall weight or the whole wall weight. It seems that it depends on personal preference.

Please let me know what you think.


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## Casey

MOOK said:


> Please let me know what you think.


I think Williams is an idiot that just paraphrased a bunch of other text books to create his wonderful error-filled series of PE exam books and has no idea what he is talking about.

Every now and then I come across something in his books that really annoys me. This week it was in reference to ACI Eq(21-8) where he says "special boundary elements are required when c is *not less than *the following"

I fucking hate double negatives!!

Would it have been too difficult to simply say "greater than"?

Anyways, I'm sticking to half walls for my tributary weights...


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## teda

In my opinion, it will depend on the location where you are interested in calculating shear force in the wall. If you are looking for shear force in the middle of the wall, half height need be used. If you are looking for shear forces at bottom of the wall, you need use full height. Hope this can help.


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## Hromis1

Casey said:


> I think Williams is an idiot that just paraphrased a bunch of other text books to create his wonderful error-filled series of PE exam books and has no idea what he is talking about.
> Every now and then I come across something in his books that really annoys me. This week it was in reference to ACI Eq(21-8) where he says "special boundary elements are required when c is *not less than *the following"
> 
> I X#@# hate double negatives!!
> 
> Would it have been too difficult to simply say "greater than"?
> 
> Anyways, I'm sticking to half walls for my tributary weights...


Casey, I having the same general problem with a series of questions. However I am trying to look at it as what would NCEES ask. So far in the problems from NCEES, they appear fairly consistent in the seismic load cases. Half hgt for element/diaphragm loads. Full hgt for base shear/overturning.

The William's books are ok and generally get the job done, but they miss a lot of details (or make some strange assumptions). I am guessing that many may really be "ghost written" by a bunch of grad students. The bridge section of the SERM appears to be particualarly poor.


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## Casey

Hromis1 said:


> Casey, I having the same general problem with a series of questions. However I am trying to look at it as what would NCEES ask. So far in the problems from NCEES, they appear fairly consistent in the seismic load cases. Half hgt for element/diaphragm loads. Full hgt for base shear/overturning.
> The William's books are ok and generally get the job done, but they miss a lot of details (or make some strange assumptions). I am guessing that many may really be "ghost written" by a bunch of grad students. The bridge section of the SERM appears to be particualarly poor.


All in all William's books do do the job. But you would think a little more effort would be put into finding errors and making statements more clear. Seeing as these are books written soley for the PE exams. Does he/his publishers not think that people are going to pour through these books in great detail.... I guess that is what gets my goat...

As for the SERM, it is the least used of my Williams books...

As for using full height for shearwalls, my wood text book suggests that using the full height is simply a conservative choice, but then again I have not found anything conclusive about which direction to take. In the exam I'll be sure to state the tributary height I am using and give my reason why. I think that should be sufficient...


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