Tall Wall - Arching

[slickdeals]

One of my colleagues has a building reviewer requesting something interesting. Thought I might run it by you folks.

A 16" thick concrete shear wall is part of a lateral force system on a 8 story building. The wall is 20' long and 90' tall. It is supported on piles as shown in the sketch.

The building reviewer is suggesting that we should do a strut-and-tie analysis of the wall to ensure that it can span between the piles on each end. Has anyone ever come across this to be done?

We are Virginia Tech
Go HOKIES

 

[dcarr82775]

It is intuitive (to me anyway) that the 90' tall wall is capable of spanning 20' between piers. The connection of the walls to the pier cap and pier cap to piers is a different story.

 

[msquared48]

This is ridiculous in my opinion.

Was he building reviewer strutting aroung in a tie?

Sounds like a make work to justify my position problem here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IDS]

I'm not a buliding structures person, but I don't think it is in the least ridiculous, any more than it is ridiculous to analyse the tensile stresses in the anchorage zones of long beams.

It surprises me that it isn't done as a matter of course.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dhengr]

Slick:
Exactly what methods or approach you use to analyze that wall is really none of the plan reviewer’s business, unless he’s signing your drawings, or unless he can point to places where your design doesn’t meet code. Most codes say something to the effect ‘analyzed by rational methods, in accordance with well-established principles of mechanics,’ and he really shouldn’t be dictating that approach. You’re running an engineering office, not an educational service. If he’s interested in how strut-n-tie design works, maybe he should pay to take a class instead of having you show him how the calcs. are done. Next thing you know he’ll want you to route out the useless conc. btwn. the struts and the ties. The strut-n-tie concept has been being batted around for years, and these kinds of conc. shear walls have been being built just about like you show yours for a long time before strut-n-tie came back into vogue. This is a shear wall, not a moment frame. Obviously, this should be handled more diplomatically than I can muster.

I agree with dcarr. What arching? If you have the right amount of shear and min. temp. reinf’g. in each face of that wall, it and the pile cap will hold up (intact) any conc. which might try to act as if arching, it all acts as an integral unit. I agree with him too regarding the load path from the wall edges through the pile cap and into the piers.

 

[slickdeals]

Dh/DCarr,
I totally agree with you guys, but I wanted to make sure there wasn't anything else.

@IDS:
Are you suggesting that everyone should do a strut-and-tie model for a wall such as one shown in the sketch? Doesn't that seem a little too much?

We are Virginia Tech
Go HOKIES

 

[IDS]

Are you suggesting that everyone should do a strut-and-tie model for a wall such as one shown in the sketch? Doesn't that seem a little too much?

I'm suggesting that some sort of assessment should be done of the tensile stresses at the base of the wall, and a strut and tie analysis seems to me like the simplest way to do it.

Maybe in practice it's never a problem, I don't know, but a five minute check to be sure doesn't seem that unreasonable to me.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

"Spanning between the piles" is clearly not an issue, by inspection. But the shear wall should probably be considered a non-flexural element, and is thus best analysed by strut-tie methods. Confinement of the struts would be my emphasis.

 

[csd72]

It is a simple calculation, I would just do it and be done with it.

 

[slickdeals]

The simplest STM model would be a triangular trusses spanning between floors (say 1st to 2nd, 1st to 3rd ... 1st to Roof) fitted inside the wall. Somehow the aspect ratio bothers me.

I am curious as to how the STM model would look like. Would a braced frame like STM make sense?

We are Virginia Tech
Go HOKIES

 

[ToadJones]

Not my area of expertise at all (not sure one exists ) but a 90' deep beam should be able to span 20'.

 

[slickdeals]

Toad:
You are preaching to the choir.

The wall is picking up loads at every floor along the way down. How much of this tall wall will act as a beam? Not all 90' of it, no? So it is more of a pseudo column that becomes a beam somewhere along the line, but where?

We are Virginia Tech
Go HOKIES

 

[ToadJones]

Maybe just do a STM from the first floor down....I would think that would still leave you a very deep beam loaded by all floors above.

 

[slickdeals]

See attached screenshot. It appears a compression strut is forming 1st floor down.

We are Virginia Tech
Go HOKIES

 

[dcarr82775]

IDS,

It says you are civil/environmental. I'll assume you are civil for my example. Lets say you have to design a culverts for a very small drainage area and in your experience you know a 12" pipe will be fine. Are you going to do the set of calculations for the 12" pipe or are you just going to put in a 12" pipe because you know it works. I would do the later if I remembered anything about how to size a drainage culvert (I don't).

Some things are obvious and don't need calculations to support them. The answer to the original question posed, is obvious.

I tend to be less diplomatic to plan reviewers. I'd tell him 'No'

 

[ron9876]

The pilecap is assumed to be a rigid element right?

Slickdeals if you model like you show then I would say S&T would be appropriate and needed--like a shearwall on caissons or a tiltwall on piles--but I don't think that is how it will function.

Aspect ratio is 4.5 but this is a cantilever and I don't know what the criteria is for a cantilever beam. However the code says that if you have a load within d then you need to consider S&T seems to apply even if it doesn't make sense.

 

[slickdeals]

A FEM model of this wall indicates that the load is purely compression till about the 2nd floor and below this the wall acts as a "transfer beam" spanning between the piles. I can intuitively understand that this could be reality because I don't expect this 90' tall wall to behave as a 90' deep flexural member.

The design of this wall for lateral forces is not under question. The design of this wall to support gravity loads while spanning between the piles is the problem (??) at hand.

We are Virginia Tech
Go HOKIES

 

[dhengr]

Slick:
The structure doesn’t know or care what you think about aspect ratios. What it knows and sees is that the outer edges of the wall are 20' apart, that these vert. compression or tension struts may be 3, 4, 5 or 6' wide and 16" thick, a function of how much vert. reinf’g. you can pack in that width and confine properly so the strut acts properly, within its limits. Otherwise, there must be a strut or tie at the fl. levels because that’s the primary point of load input. The structure understands (knows, dictates) those two things (wall dimensions and fl. elev’s.) and they set the configuration and general dimensions of your strut-n-tie system, if you choose to use this method of analysis. This is essentially what we have always been doing and thinking when we designed these walls, without specifically calling it the strut-n-tie method or having all the fancy new rules and code reqr’mts. The last one of these that I did consisted of two vert. walls about 26' wide by 12" thick, about 200' tall, both in the same plane. The two walls were coupled together at every fl. level by the fl. slab and a wall/beam (integral Tee bm.) which crossed a central corridor in the building. I didn’t think to call it strut-n-tie, although I knew of some research on that subject at the time. But, I’ll bet if I went back and looked at those designs and reinf’g. they would come pretty close to what strut-n-tie req’rs. today. And, the bldgs. are still standing and functioning just fine, smiling at me every time I drive by. I think hokie said or meant about this same thing @ 21JUL, 20:31, in fewer words. And, I still agree with dcarr, the difficulty is tension from the wall edges into the piers.