bending moment bars at column to roof slab 7

[ajk1]

Question about the horizontal length of the "L" shape bars between the column and the roof slab, that are there to transfer unbalanced moment between the column and slab: Should that horizontal length be a tension lap length of the L bar with the top bars in the roof slab, or should it be a tension development length? See attached.

 

[Celt83]

Gotcha, yeah I realized right after I posted that you'd end up with unreinforced tension struts at the orange dashes with not insignificant forces.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[ajk1]

Thank you for the diagrams. A picture is worth >> a thousand words. Could have saved time if that was provided at the start. Thank you again for your help.
For most practical roof slab designs, it would be interesting to assess whether that is a real failure mode. Has that ever been observed in the lab?
We rarely if ever do a roof design without drop panels, just because of architectural & client requirements for the spans required in modern construction, deflection limits, snow load and snow load drift weights, punching shear limits, etc. Also there are bottom bars ("structural integrity") bars in the bottom of the slab that must by Code extend over the column, which may help resist the breakout a little, although not the tension tie required in strut-and-tie model.
Thanks again for your help. We will give further thought to it before finalizing any revisions to our typical detail.

 

[BAretired]

Great discussion! At 113 years of age, I'm amazed KootK can still chew a steak!

BA

 

[KootK]

You may have to pre-chew my food for me. It's uncomfortably intimate, I know, but it seems like a small ask relative to a thousand stars.

 

[ajk1]

I do not understand what the significance of the 113 years is, its relevance to the question, or anything that I said, but never mind, probably something very sophisticated. You seem to be nearing 120 years (i.e. the age Moses lived to be). Great photography. I like that. Always wondered how birds and beavers and bees and other creations know the engineering of how to put together a nest or a hive or a damn, etc., and strut-and-tie theory. They did not learn it in school.

I was wondering where BARetired was. Good to see you back.

 

[ajk1]

How does buddy16 work?

 

[ajk1]

How do you post something directly into the post, as you did with the photo and diagrams(i.e.not as an attachment)? Do you just click on copy and then paste?

 

[KootK]

I do not understand what the significance of the 113 years is, its relevance to the question, or anything that I said, but never mind, probably something very sophisticated.

Not at all sophisticated. I was just being facetious regarding the nature of extensive experience.

How do you post something directly into the post, as you did with the photo and diagrams(i.e.not as an attachment)? Do you just click on copy and then paste?

My procedure:

1) Bring the image that I want up on my screen at about the size that I want to see it when I post.

2) Use Windows' native "Snipping Tool" to take a snapshot of the image.

3) Save the image somewhere convenient.

4) Come back to eng-tips and select that little square picture frame icon in the tool bar above the text box where you type.

5) You'll be asked to select a file and you'll select the one from step #1. Until you post, the "image" will just appear like the string below.

{img httpq://res.cloudinary.com/engineering-com/image/upload/v1556198435/tips/c02_l8ombx.jpg}

 

[Celt83]

How does buddy16 work?

Assume your asking about the programs. You'll need to have Python 2.7 and the following modules at a minimum matplotlib, numby, scipy, and itertools. Alternatively you can install the 2.7 version of Anaconda. After you have all that installed you can download a copy of everything I've put out there by clicking on the green "Clone or download" button and choosing download (you might need to have a github account for the button to show up but I'm not entirely sure on that)

Once it's all downloaded and unzipped any file that has "GUI" in it's name can be run by just double clicking the file and you'll get a user interface like the pictures on the github page. Anything without "GUI" in it's name I highly recommend opening in Spyder if you chose to install Anaconda, that will give you the ability to both see the code and also run the script.

Understand that all of the programs out there are ever evolving and may contain bugs either indicated on the github page or that I have not found yet.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[ajk1]

Kootk - thanks for the clear instructions. Much appreciated.

Celt83 - thanks for the info. Much appreciated. Someday I will try it.

 

[KootK]

A little serendipity at work tonight. I was digging around in the KootK archives for something and stumbled upon this. It's exactly what what I was harping about above for an improved detail.

 

[EZBuilding]

Would you provide enough #5's to have at minimum 1% steel for the column?

 

[KootK]

Nope. I can't see any rational basis for that.

 

[jayrod12]

So you would just provide slightly more #5 dowels than required by analysis for the moment transfer your expecting. Certainly reduces congestion.

I'm going to take a look at our typical details now and see what we show. Perhaps recommend some changes if required.

 

[ajk1]

Save me from again reading thru this lengthy string of comments, but is there any reason for providing anything more than a standard hook at the end of the dowels, since added length of hook does not increase the force that can be developed in the bar...that is governed by the crushing of the concrete at the 90° bend. So why do some details say the horizontal length should be a tensile development length.

 

[KootK]

I don't see any rationale for the tension development in the horizontal bar projection unless the bars are crossing over the column.

 

[ajk1]

ok, thanks.

 

[ajk1]

I wonder if it is possible to set a limit on the hooked bar size together with the minimum slab thickness that can be used that will develop the hooked bar without the sort of corner breakout that you illustrated in your earlier post in this string. Although it may be difficult to develop an exact formula for this, perhaps it is possible to develop a very conservative expression for it. Say for a 20M (≈#6 bar) and 200 mm or 250 mm thick slab. (I don't think > 20M bar should be used in a flat slab anyway, so that would be the max bar size). Could assume a geometric failure wedge and a permissible conservative shear stress along the surfaces of the wedge. Then could limit when the bars would be turned outward to conditions satisfying this; otherwise the dowels would be turned inward.

 

[BAretired]

One possibility is to use an end anchor for the column verticals and run straight top bars in slab just below the anchors.

Another possibility is to use serious ties between column verticals at bend point to allow bars to bend outwards, yet prevent breakout.

BA

 

[ajk1]

to BARetired: Very good ideas, as is usual from you. I still feel that for practical designs, in real life situations in the area I am in, with the snow loads we deal with etc., there is no real problem with corner breakout, but in some unusual case where there is an ultrathin slab and large diameter bars (i.e. not a good design) then your solution would be a very good one indeed.