Checking web penetrations in existing concrete beams

[Erarius]

Hi all,

Do you have any guidance on how to approach checking existing concrete beams for web penetrations?

If the opening just misses the stirrups (i.e. within the space between stirrups), is that considered acceptable (if the penetration is small enough diameter ~ say 0.3 x depth of beam)?

I understand the traditional method is that there is a vierendeel moment induced over the span of the opening which equals to (V*top/bot) x (Opening Length) / 2. But inside an existing beam you would not have longitudinal reinforcement directly above or below the opening and you won't be able to put it in obviously. So theoretically speaking, there isn't anything to resist that vierendeel moment (regardless of how small the penetration is)


So theoretically speaking, even a 5mm diameter hole cannot be allowed in a 1000mm deep beam. So what is the reasoning why this isn't the case?


Thanks

 

[Agent666]

I don't know what code you are working to, but often there are exceptions in codes for smaller holes of a certain area provided they are located in a certain location in the beam.

So in my view only these smaller sized holes are probably acceptable to add to an existing beam because as you note you cannot get any more reinforcing in there where its needed. You might be able to instead put some external reinforcement like carbon fibre/FRP, etc to do the same thing as internal reinforcement.

The reasoning being for larger holes requiring more thought and specific design checks is you can get the following type of failure which is often reinforced for in a new beam via additional smaller stirrups above and below with additional longitudinal reinforcement just above and below the opening, or via diagonal reinforcement going under and above the hole. But in an existing beam you can see how you can get a failure plane that doesn't intersect any of the shear reinforcement, which means you effectively can end up with a section that is unreinforced for shear.

 

[Tomfh]

It's not really a vierendeel moment. Provided there is enough concrete (and steel) left to carry the bending and shear at the critical cross section then it works ok.

We typically allow small penos in the middle third of the beam depth. Once the penos get bigger/longer and more numerous it gets more complex.

 

[Agent666]

Tom, most codes I'm aware of require the designer to consider the secondary moments across the opening and may therefore require additional longitudinal reinforcement adjacent to the opening to both anchor any additional stirrups above/below the opening and to deal with the vierendeel moment. As shown in my sketch, the critical cross section can have zero shear reinforcement being engaged, so its important to check what type of failure might occur (see screenshot below for the two possible types, beam and frame type failures).

For some configurations, an alternative means is to demonstrate that there is a strut and tie arrangement that is capable of sustaining the shear across the opening. For example if in my sketch the beam was much deeper, then even with the assumed location of the shear plane you can hopefully cross at least 1 to 2 stirrup sets either side of the opening for a beam type failure like shown below. In that case you can probably demonstrate that it works if no forces were required to pass through where the hole is. Really depends on the geometry/hole size I guess.

Following shows this concept and the two type of failures that need consideration (from Concrete Beams With Openings - Analysis & Design - Mansur, Tan (1st Edition - 1999))

 

[Tomfh]

If the hole is so big that it creates significant secondary moments then sure, but for little penos in the middle third of the beam that don’t interfere with reo (eg a 100dia pipe central in a 600 deep beam with ties at 200) it’s not significant.

 

[Erarius]

Thanks guys.

So are we saying here that:

If the penetration is small enough diameter, that a 45 degree crack as shown in Agent666's sketch will have to cross a stirrup, then it would be okay?

Currently dealing with a 750 deep beam with a 150 diameter hole. The stirrup need scanning but probably at 250mm spacing.

The codes are all very vague about this kind of thing and I'm really struggling to find a reasonable approach in terms of analysis. Because technically speaking, a 5mm hole is not even allowable haha..

 

[Agent666]

You've still got to check it, for one you're loosing 150mm of depth in consideration of the concrete shear component, and your effective depth/stirrup spacing ratio is effectively reduced which also affects the steel component. So the number of sets engaged by the frame failure type of failure is less sets than elsewhere in the beam.

If your frame is subject to reversing seismic loads or worse requires capacity design & overstrength forces then putting a hole in and reducing the shear capacity locally might be creating a point of failure in shear which is not desirable.

I'd recommend seeing if you can get the text I referenced as it pretty comprehensive. Codes generally state something along the lines that you must use a rational method of analysis, so pretty much anything goes. But there are recognised ways of dealing with the design, such as the reference I provided.

 

[Tomfh]

Just assess it. Check cracks at 45 degree, see how many stirrups you have to hit, and work out capacity.

Where’s the Peno? High shear zone? Or low shear?


And can they truly core it without hitting anything?

 

[Erarius]

Thanks Agent666 and Tomfh.

Yes I understand, and I will be checking it. I have actually read through multiple literature including the one mentioned before I posted on here.

The checking of the stirrups part I get - shear/tension in stirrup times number of stirrups it would cross at a 45 degree

But it's the secondary/vierendeel moment part that I can't get my head around. This is key component that I'm not getting out of literature..

Hope you guys know where I'm coming from.

 

[Yao1989]

https://www.researchgate.net/profil...747/download/Concrete+Beams+with+Openings.pdf

This is the best resource. May have some test results.

How small is small enough? I can only suggest try drawing a struct and tie and see how small before the effect of the opening can be neglected. Sometimes you get lucky and your compression strut capacity is not affected by the area lost from opening?

That said for 150 opening in 750 beam I am almost certain you will need some strengthening for vierendeel moment..