Stud rails for punching shear 2

[la belle vie]

I just took over a project involving the design of a long elevated outdoor pedestrian ramp. The design is over 90% complete and consists of a flat slab supported on round concrete columns. The calculations show that the slab works in punching shear, but just barely. To minimize risk I want to add some stud rail shear reinforcing. Without this, my concern is that any cracks that may show up in the slab around the columns could make people nervous about progressive punching shear failure. Stud rails just seem like cheap insurance. The client, a public entity, has a lengthy approval process for propriety items, which brings me to my question: Would it be crazy to detail “stick built” stud rails myself and let the contractor propose ready made ones if he or she would prefer not to stick build? Any advice would be greatly appreciated!

 

[KootK]

1) you'll get the cracks with or without the stud rails. They don't activate until cracking.

2) Personally, I wouldn't bother with the stud rails. I love me some non-congested concrete that works on its own in shear.

3) How thick is the slab? Depending on your answer, tie or shear head reinforcing might be a more suitable DIY reinforcing strategy.

 

[la belle vie]

Thank you KootK. The slab is 12 inches. Cover on top is 3" and 1.5" on the bottom. Columns are 15" in diameter. DIY rebar would definitely be easier to specify. Some others here are worried about constructability. Shear heads would probably raise eyebrows.

 

[ggcdn]

For what it’s worth, I often find myself adding punching shear reinforcing when the design technically works but utilization is close to 100%. But I’m in a seismic zone and there is the added uncertainty of the connection degradation as the building displaces.

Why so much top cover? Is that standard for pedestrian bridges?

At 12in, with 1-1.5” cover, you could use stirrups for punching shear without issue. On thinner slabs, or with such large cover, the effectiveness of the stirrup comes into question. Though, if you are doing this for feel-good measures, maybe that’s still a good compromise.

-JA
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[la belle vie]

3" cover is the DOT's standard for bridge decks around here. For protection against deicing salt I think. With stud rails, I was thinking to let the tops of the shear studs only have 1.5" cover.

 

[KootK]

Yeah, I like to be around 16" before I use ties and even that would be for interior covers.

I've reviewed a number of delaminated slab soffits in the past where there were studrails. Often, the rails end up encroaching upon the design cover significantly, either because of a chairing fails or because too much weight was applied to the stud rails and they drooped between chairs. This is one thing that makes me a bit leery of using stud rails in an exterior, potentially corrosive environment. I see the odds of a punching shear problem as being relatively small for your setup and not worth the risk of a potential durability problem.

Let's say you'll do the DIY studs:

1) Sure, I would expect them to work quite well in a non-durability sense.

2) So long as you're not using the rails for ULS capacity -- and you say that on your drawings -- you should be able to get by without an ESR report etc.

3) I'm not sure if you risk any potential to run afoul of somebody's patent. Not that you're likely to get caught either way.

 

[hokie66]

Like others, I wouldn't use stud rails in an exterior situation. But then, I have never used them anywhere.

For a pedestrian ramp, I can't see adhering to 3" cover as required by your DOT for bridge decks. Moving the steel to where it is needed would be my choice. A typical problem with top bars is using the wrong chairs, resulting in your bars being lower than required, so reducing that 'minimum' cover would be a positive step. Another suggestion is to up the cement content of your concrete, which provide better protection. And pay special attention to compaction, particularly around the columns.

 

[la belle vie]

Really appreciate the food for thought. I’m going to triple check the calculations and seriously consider doing without shear reinforcement.

 

[ARKeng]

Have you considered playing with your concrete strength? Depending on what your starting f'c is, a 500 or even 1000 psi increase can be had fairly inexpensively compared to cost of rails or shear reinforcing (especially when you start considering not just direct cost but submittals, proprietary issues, lead time, shipping, installation, schedule effects, secondary problems like congestion, etc.).

Can often buy you that extra "sleep insurance".

 

[cliff234]

It sounds like your slab is exposed to the weather and de-icing salts. If so, is your reinforcing steel epoxy-coated? And if your rebar is epoxy-coated I would think the studrails would have to be similarly epoxy-coated.

If your slab is exposed to weather, will you have drains near the columns? If so, there may be water near the columns. The top of the slab will be in tension near the columns, cracks may occur, and you may get corrosion in the studrails if they are not epoxy-coated.

Based on my experience, 15” columns seem small for a 12” slab. Is there any chance you can make the columns larger? If you use 24” diameter columns, you’ll get an appreciable increase in punching shear strength without having to resort to studrails.

Finally, did your punching shear calculations take into account shear stresses due to unbalanced moments (from wind or gravity loads) in the slab at the slab-column joint?

 

[dik]

I've never used stud rails... I've never had the confidence in them. Also, for narrow columns, I add a couple of 25M (#8) bot extra in the slab, through the column with ten development length outside, in the direction of the long dimension... not a code requirement, but my own code.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[la belle vie]

cliff234- You have hit on all the major issues. The columns feel small to me too. The rebar is epoxy coated, but there are no curbs and drains. There will definitely be salty water near the columns. The punching shear calculations did take moments into account, but I just have a bad gut feeling. I am the PM, not the EOR and he doesn't share my concerns. Will look into upping the concrete strength, reducing cover and adding more through steel. Thank you all!

 

[hokie66]

You are welcome, I'm sure.

 

[dik]

Some MOTs are moving away from epoxy bars due to added corrosion issues from breaks in the coating... you might want to look into that.


That's one of the last things you want to do if there are corrosion issues...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[cliff234]

La belle vie - Is the EOR not concerned because he has designed dozens of flat plates - or because he has designed only a few? The older I get, the more I realize how many things can go wrong in our profession. I sometimes use the analogy of a clueless person walking through a minefield. If you don't know you're walking through a minefield, then you'll just stroll through happily. If you know your walking through a minefield, you'll be terrified, and gently probing every square inch.

One thing to keep in mind. Make sure that when they pour the columns that the top of the column pour is flush with the underside of the slab. I have seen many times where the column is poured a couple inches too high - and if it is not chipped down, then the punching shear strength will be seriously diminished. We always tell the contractor and the special inspector that the top of the column pour must not encroach into the bottom of the slab. If this mistake is going to be made, it will often be made on the first column pour - and that's when it needs to be caught.

 

[hokie66]

cliff234,
Good point about the correct height of the cast columns. Worst thing for punching shear is to cast the columns too high.

dik,
Point taken about epoxy coated bars. I don't have much confidence in them. On the other hand, galvanizing is great insurance.
The top cover specified is 3". Ridiculous for a 12" suspended slab. Any cracks which form will be widened due to the depth of the bars.

 

[la belle vie]

Great advice all around. I would love to change to galvanized rebar instead of epoxy. It really wouldn't be too difficult change all the (E)s to (G)s even though 100% is in less than 3 weeks away. I am going to try to reduce the cover by one inch too. Part of the 3" for bridges is for sawcut grooving, and we are only turf dragging. Awesome point about the column heights. I was more focused on the top of the slab. I wonder if the columns could be poured 2" short and have 2" fillets all around as extra insurance.

I am new at this company and don't know how many flat slabs the EOR has designed. He's very confident, while I am more the terrified type. I have never designed a flat slab, being more of a bridge person, but I had the fear of god put in me over the whole Florida thing.

 

[hokie66]

Fillets for a circular column generally requires special forms. A small square capital would accomplish the same thing.

 

[dik]

Hokie... HDG is great, almost as good as stainless, depending on the environment...


I've often use 1-1/2" or 2" for 6" parking garage slabs... going back nearly 40 years ago... long before it was common.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik