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Punching shear (heavy loading warehouse)

Punching shear (heavy loading warehouse)

Punching shear (heavy loading warehouse)

(OP)
I have a tender warehouse design with heavy loading (50kPa).The panels are square (12m x 12m). How do you resolve punching shear outside the drop panels? What will be the best economic way? Is there any other system for punching shear other than stud rails or links.

Thanks

RE: Punching shear (heavy loading warehouse)

This is PURE Structural Engineering...

Are you sure you wouldn't be better off with the help of a local S.Eng.?

As for your question, this is an area of fairly active research, and codes differ widely in their applicable clauses. What code are you working with?

In CAN/CSA A23.3 you would resolve this as a slab thickening and ensure you had enough thickness and overall size to create a perimeter with sufficient shear capacity to handle the shear load at the transfer point. That said, the fact that the soil is there underneath is first taken into account to "credit" out the bearing capacity of the soil. Additional load is taken by the "skirt" through regular slab thickness once the load is within the permissible range for the soils. Therefore you're looking to get a slab thickening large enough to spread the load down... It is very straight-forward Structural Engineering, but an area where different Engineers and Codes/Standards require/implement very different (analytical) solutions.

RE: Punching shear (heavy loading warehouse)

CELinOttawa - I think this ia a suspended RC flat slab, not a soil-supported concrete slab on grade.

struggle66 - please confirm.

RE: Punching shear (heavy loading warehouse)

Ingenuity: You're quite possibly right... I did assume that this was a slab on grade, but if this is a suspended slab there is no way anyone but an experienced structural should be designing the slab...

All of my comments still apply, aside from deducting soil stress.

Don't do this on your own if it is a suspended slab... Bad enough if a slab on grade, but for a suspended slab screw-up people are very likely to die.

RE: Punching shear (heavy loading warehouse)

Yep whatever happened to asking someone senior in the company or looking at previous projects.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Punching shear (heavy loading warehouse)

Slab thickness...short answer. 50kPa live load, 12m bays, probably outside the range of practical flat slab design. Needs a good structural concept design before any "tender" consideration.

RE: Punching shear (heavy loading warehouse)

AS hokie noted, if concrete, you are likely outside the economical limits for a flat slab design. You may want to look at beam and slab if concrete or composite steel beam and concrete slab for steel 12m composite girders and purlins at 4m spacing or something of that ilk...

Dik

RE: Punching shear (heavy loading warehouse)

(OP)
Hi all
It is a suspended PT flat slab.

Yes I understand that I dont have experience and should be working under a mentor but everyone dose not have the privilege of working with a mentor.
And my company is only a PT sub-contractor so there is no one to ask. I better find a new company or quit my career :D.

Anyway the structural consultant insists flat slab with drop panel so I have no choice.

I got slab thickness 550 mm and drop panel thickness 1100 mm(5m x 5m) but a lot of stirrup need to put both inside all drop panels and outsite all drop panels.

I use 70% and 30% moment distribution between column strip and middle strip for both directions but here in Singapore almost everyone uses average moment method (i.e.. there's no middle strip and column strip). So normally my quotation can not compete with others.


RE: Punching shear (heavy loading warehouse)

"everyone" is wrong. Your thicknesses seem about right for a flat slab, but you should not be placed in this position. The "structural consultant" should define the concrete profile.

RE: Punching shear (heavy loading warehouse)

Agreed with Hokie, everyone else is wrong. The average moment concept is bad enough for flat plates, it is completely wrong for flat slabs with drop panels. Significantly underestimates the overall slab strength.

For a slab this thick you could use normal shear reinforcement but I would prefer more concrete!

RE: Punching shear (heavy loading warehouse)

Quote (hokie66)

The "structural consultant" should define the concrete profile.

I will go further and state that for final designs, the structural consultant should be fully designing the PT floor system.

I guess fees must be depressed such that many consultants now deflect the design and responsibility to the PT sub contractor. Most common in USA, Australia and Asia, evidently.

I have been on the other side of completed PT structures that had major problems due to this format for design/construction.

RE: Punching shear (heavy loading warehouse)

I agree completely, Ingenuity. The division of responsibility almost inevitably leads to problems, both of administrative and structural nature.

RE: Punching shear (heavy loading warehouse)

For such heavy loads, a PT system could be interesting if you have areas that are unloaded. For most industrial work, I tend to shy away from PT because of the potential for abuse and alteration.

Dik

RE: Punching shear (heavy loading warehouse)

(OP)
Hokkie66, Ingenuity
I agree with you. The problem that I quite often encounter is during construction when there is an structural issue, main-contractor always try to ignore structural integrity. We sub-contractors can not force main-con what to do even though we designed it and it become worse when structural consultant try to stay away from responsibilities.

RE: Punching shear (heavy loading warehouse)

(OP)
Dik,

Are you talking about pattern loadings?

RE: Punching shear (heavy loading warehouse)

Yup... even the installation of equipment may present a pattern type load... If you have very high loading, then with PT you have to accommodate it some how... even a pile of regular reinforcing. I think your thicknesses are of the right magnitude and your 30%-70% distribution is reasonable. I'm just not so sure that system is correct for post-tensioning. Can you use composite steel? Has anyone investigated the use of this?

Dik

RE: Punching shear (heavy loading warehouse)

Dik,
I disagree. With his spans and loading, I think post-tensioning is inevitably part of the solution. Partial post-tensioning, so your "pile of regular reinforcing" will be there as well. A structural steel solution could be considered, but the OP works for a PT company.

RE: Punching shear (heavy loading warehouse)

Dik,

Not sure where abuse and alteration come into it, but general code defined pattern loading analysis is required to handle this plus any known or possible pattern of loading must be investigated as well.

For heavily loaded PT members, partial prestressing should be used. It provides a very good result in terms of section depth, crack control and deflection control. There is no logical reason why a PT design cannot have a large amount of reinforcement also, it is always required for heavily loaded members like warehouse slabs and transfer slabs/beams.

But using the average moment method for this type of design is not possible. ACI is the only code that allows average moment design (the others do not because the logic is rubbish and it is just plain wrong (I await heaps of abuse from unbonded PT designers from USA)) and even the ACI code does not allow partial prestress design using it. The only other document that discusses it is TR43 from UK which puts significantly reduced allowable stress limits on negative moment regions for this type of design, reflecting the gross inaccuracy and illogic of the method, and does not allow these limits to be exceeded by adding extra reinforcement for crack control. Effectively both documents do not allow partial prestressing to be used with average moment design. So the design method cannot logically be used for heavily loaded slabs. So all of the PT companies in Singapore doing this should stop it and start designing in accordance with the codes and TR43!

RE: Punching shear (heavy loading warehouse)

(OP)
Beside warehouse there is also ramp to the warehouse for heavy traffic (L.L = 20 kPa). Consultant gives me the ramp layout plan(Kindly see the attachment). And I believe tendons shall never be bent like that so I plan to run the tendons straight as per the attached sketch. Is it usual in practice? When you design this kind of ramp what is your usual plan? I dont have experience in ramp design with PT beams. Please help me advice.

And thanks for your reply. Actually I shouldn't be disturbing you with the basis of design.

RE: Punching shear (heavy loading warehouse)

Looks like long, simple spans with a 'slab band' approach... thanks for the clarification.

My experience with industrial constructions is that over time they get modified and or changed use... and there can be a problem due to the embedded strands.

Dik

RE: Punching shear (heavy loading warehouse)

(OP)
dik,
It is one way PT slab with PT Bands/Beams. The problem is the tendons can not be bent at the curve portion of the ramp so I ran the tendons straight( I sketched the tendons on the plan).Maybe it is not clear.

RE: Punching shear (heavy loading warehouse)

struggle,
My approach for the ramp structure would be to use partially prestressed band beams, but use only bars in the slabs. You are correct, curving strands around that ramp is not the solution.

RE: Punching shear (heavy loading warehouse)

struggle66,

The pattern you have suggested will work. You just have to resolve the capacity of the tendons in the 2 directions to your design direction, similar to analysis where you get Mu and Mv as the principal moments but resolve them to Mx and My to get orthogonal moments for design.. You will however have trouble with the anchorages around the curve. Some well placed construction joints might make it possible to stress mainly from square ends rather than curved ends.

You could curve the tendons around the ramp using round duct multi-strand stressed tendons if you want to. Friction will be higher but that can be accounted for and multi-strand tendons typically have less friction than flat duct slab mono-strand tendons..

RE: Punching shear (heavy loading warehouse)

I think Hokie's got it using strand for the slab bands and for the straight portion of the slab and using conventional reinforcing for the curved end portions...

when you were mentioning the proportions of post tensioning for the column strip middle strip, I was not thinking about the configuration shown.

YOu may want to look at the column size and try to minimise the depth of them and make them wider to attract less moment.

Dik

RE: Punching shear (heavy loading warehouse)

(OP)
Dik,

I mentioned about column strip/middle strip for the warehouse design. Now the plan is for the ramp to the warehouse and if the columns attract less moment, the PT beams design will become worse.

Rapt,
Can rapt software account for that kind of friction losses?

One more another topic about Singapore practice

I know that bonded PT can count for minimum reinforcement but the spacing of tendons is so large. Is it Ok without min un-tension reinforcement between tendons? The practice in here is that they just put BRC A 10 for slab thicker than 325mm, A9 for 275, A8 fro 250 whether it is required by software which count bounded tendons as untension reinforcement or not. What is the good practice?

thanks

RE: Punching shear (heavy loading warehouse)

The slab bands will likely accommodate greater moment (due to their T shape and greater depth) than the columns...

Dik

RE: Punching shear (heavy loading warehouse)

struggle66

RAPT allows the designer to nominate an extra angle change in each span. This is assumed to be transverse to the normal drape angle change and is included combined with the drape angle change based on the sgrt of the sum of the squares of the 2 lots of angle change.

In Singapore code,
if the cross-section is class 1 or 2, the prestress bonded tendons are assumed to provide adequate crack control, as long as all stresses are accounted for. The actual steel stresses in these cases will be very low so crack control is not really a problem. If there is strong restraint to shortening (which is often not included in the stress calculations) then extra un-tensioned reinforcement should be added.

For class 3 members, maximum bonded reinforcement spacing should be 300mm.

RE: Punching shear (heavy loading warehouse)

struggle,

My comments above on crack control assume average moment design approach is not used if it is a flat plate or flat slab!

RE: Punching shear (heavy loading warehouse)

(OP)
Rapt,
Thanks and sorry for taking so long to get back to you.

You mentioned that " For class 3 members, maximum bonded reinforcement spacing should be 300mm". Normally the bonded tendons spacing will be around 1 m ( < 8 times slab thickness). How can I compute the size of normal un-tensioned bonded reinforcement between bonded tendons(parallel to tendons) to satisfy 300 mm spacing.

And I had a look at TR 43. According to TR 43 clause 6.10.6,

"there are no minimum un-tensioned reinforcement requirements for one-way spanning floors with bonded tendons. It is considered that these floor have sufficient tendon-to-concrete to distribute flexural cracking". And it only specifies the spacing of bonded tendons should not exceed 8 times slab thickness.

For two-way, TR-43 only requires minimum un-tensioned reinforcement at column positions.

And TR-43 doesn't state whether it is class 1 or 2 or 3.

RE: Punching shear (heavy loading warehouse)

If you are using TR43 edition 2, it will not mention classes of prestress because it is based on Eurocode 2, not BS8110, and Eurocode does not mention classes of prestress.

You would use the Eurocode crack control rules which are based on the stress in the reinforcement (which is why it cannot be based on average moments). These give maximum bar size and spacing depending on the stress in the reinforcement and the crack width being designed for. The maximum spacing is 300mm.

RE: Punching shear (heavy loading warehouse)

Struggle66,

You might want to throw a couple of folks some stars for their helpful posts.wink In any case, I'm glad you weren't dependent on me for some of those answers. You'd be glad too.3eyes

Fascinating discussion.

RE: Punching shear (heavy loading warehouse)

(OP)
Archie264,

I always thank to everyone who help me out with their helpful answers. It is really helpful for an inexperience engineer like me. smile

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