DEFLECTION ON TILT-UP PANELS W/BRICK VENEER 1

[mlevario99]

We are working on a project that has tilt-up panels with Brick veneer, is for the goverment. Normally on tilt-up panels your deflection criteria is L/150, on this project they since they have brick they want to limit the defleciton to L/600, which is high for a tilt-up building, not to mention that this project is on a 130 MPH wind zone and a building category IV . This is a warehouse with typical roof metal deck and joists. The design height of the panesl go from 26' with 7' of parapet, and up to 30' high with 3' of parapet. Now, ASCE allows to use 70% of the wind load to calculate deflections, my quetion is, can I design the panel with the full wind load to calculate my steel for bending and then go back again and use the 70% wind load for deflection, since your effective moment of inertia depends on your Ma, this will make a big change in deflection

 

[ishvaaag]

Certainly the moment of inertia used for a service level check, that I am assuming the lateral deflection check is, must be for a cracked section moment of inertia for the causing loading, hence, 70% of wind load hence per the ASCE allowance.

 

[a2mfk]

That is how I would do it, but stick to your guns on the L/600 deflection for brick. Its not your fault the owner/architect selected brick on a tilt wall building instead of a stucco or painted finish...

 

[ishvaaag]

And to prove that to anyone you only have to say this is the status of the sections for the pertinent loading.

 

[JAE]

It is not the cracked moment of inertia, I_cr that should be used but of course the EFFECTIVE moment of inertia, I_e that applies to deflections.

The original question brings up a good point in that I_e must use an M_a value from a load - and the question is do you use full wind or 70% wind for that derivation of I_e? I would say use the 70% wind.

 

[spats]

Good luck meeting L/600! You do not have to assume the panel is cracked at service load. See ACI Section 14.8.4. That's the only way you're probably going to get it to work.

 

[msquared48]

Your major cracking and deflection in the panel will come during lifting, not wind or seismic. Better consider that too.

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

 

[oldrunner]

I think the L/600 limitation is for any structural system that is using brick - whether it's steel studs or something else.

 

[ishvaaag]

As JAE says it is the effective moment of inertia what to be used, per the ACI code.

In any case, as per the science of construction, one could to follow the actual status at the sections, by segments, to be taken as input in matrix analysis, and this must not be thought as some very extra complication, since (except for the hints given in ACI) one has to work starting with the acting moment, that varies with hypothesis and section.

What brings to my mind the saying of the head of one of the ACI's deflection chapter saying, well say we depend on 10 parameters, make a 5% error on each, so we can't assert our computed deflection be in 50% range of actual. I in fact also remember some people making an analysis at some corner and looking only one of the edge beams asserting some deflection; we showed them that if their computations were ok, the other beam at the corner (under 20 ft span) would have to be showing 4 inches deflection, that obviously did not.

So it comes a point where either the statistical unreliabilities (concrete) or ability to measure (actual material properties of the steel, geometrical imperfections, incorrect restraint assumptions, particularly at tests) severely impair our ability to make a proper prediction of deflections.

 

[Galambos]

you can reduce your wind load for a 10 year storm beyond 0.70 for 130 mph wind zones by using the table in the appendix of asce for higher wind speeds.

i believe it is on the order of 0.61 or so.

 

[racookpe1978]

Skipping back to the original idea: What about the brick?

I have done some bricklaying and brick repair, but I cannot figure out how your archetect is planning on laying a brick wall "sideways" on a flat surface 26 feet wide (it is a tilt-up building right?) and get the mortar and bricks to lay evenly.

How does he expect to maintain a "level" row every time repeatedly when the bricklayers are working "sideways" on some flexible surface (?) or backing plate or work platform that is going to rotate? Surely he does NOT expect to build a standard brick wall horizontally. How will he attach the brick wall to the backing member - with a full layer of new mortar? An epoxy glue of some kind? How long will he need to let the bricks and mortar to dry before that can be lifted up?

 

[hokie66]

racookpe,
Why would you build the brick veneer any other way than the normal way? I don't think the OP said anything about putting the brick on before erecting the panels. The masons would work from scaffolds, and the brick would be connected to the concrete backup with brick ties.

 

[a2mfk]

Can you do some type of stamp in the surface of the tilt that looks like brick at a fraction of the cost?

Or maybe precast brick panels that connect directly to the tilt? I've seen these on other projects but never worked with them directly.

Just seems like there is a more economical way to do this, but I suppose in the end its not much different than CMU with brick.

 

[racookpe1978]

I guess I didn't make myself clear -> If the bricks are going to be added after erection, using the usual brick-and-mortar construction, then the 1:600 deflection isn't needed "because of the brick". The brick can go up and look good over just about wall deflection or distortion. (And brick probably has hidden more serious distortion problems in millions - or tens of millions) of buildings already.)

If the brick is (somehow) laid first, then is he (the architect) requiring the 1:600 deflection just to prevent distortion/cracking of the brick as it is tilted up? Makes no sense, because the distortion from lifting/tilting up a heavy brick wall is going to be almost impossible to prevent without stiffing the wall until it will resist not only wind pressure, but tanks (and cannon and bullets) pressure. 8<)

 

[ishvaaag]

Masonry may start to hairline-crack about 1/2000 deflection. Traditional codes established limits of around 1/500, and have read somewhere that generally limits were cut at 1/600 because by such stiffness the buildings started to be on tha viewpoint uneconomical. This of course assumes masonries following the distortion, theoretically, attached.

The point here is that it seems that someone decided to place a code restriction at the upper limit of the economical scale of prevention against cracks. Respect cracks as related to distortion, will appear in akin form irrespective of the cause is erection or lateral loading.

 

[slomobile]

a2mfk is right, a good formliner and a nice paint job will get the same effect, and save the taxpayers money in the long run, both during construction, and the future maintenance. It's done all the time.

They can find another way to waste the extra cash.

 

[hokie66]

racookpe,
The deflection limitation for brick veneer is due to lateral loading in service. There are different limitations placed on this by different codes, but L/600 is common, and certainly more easily achieved with a concrete backup than with wood or steel studs.

 

[concretemasonry]

I don't know if I missed something.

Is this a a true brick veneer (thick about 3 5/8") or an adhered brick veneer (thin) that is used as a bed when the panel is cast?

In the second case, from a structural standpoint, it is a part of the panel and is controlled by the structural specifications because it moves with the concrete panel.

For an industrial/warehouse building, I would not expect to the real "Cadillac" veneer (3 5/8" thick with flexible ties) that is not connected structurally.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.