Shear key design for to support column in 14-storey building 5

[Temporaryworks]

Hi All,

I would be interested to get peoples opinion on the design of a shear key around a column which receives the load from a transfer truss supporting part of a 14-storey building. Please could forum-users read this in conjunction with my attached sketch which is intended to convey alot of the information you might need to reply. I would like any discussion on this but am interested in failure mechanisms. I am especially interested in the my interpretation of the shear failure which I would appreciate comment on.

I would undertake a bearing check between the shear key and the concrete
- As there is a eccentricity between shear key and the applied load from the truss, Macalloy bars are to be used to provide equilibrium and they would be pre-loaded to maintain a clamping force and prove the installation.
- The shear failure would be a vertical shear failure akin to the maximum shear capacity achieved when designing a beam for web crushing at supports. Thus a significant shear strength can be mobilised in the concrete - in AS3600 this is approx 0.2*f'c*do*b where b is column width and do = height of column above shear key. The column would be able to mobilise this capacity above both shear keys.
- Steel design/stiffenrs/welded connections etc obviously required
- Bars to be preloaded to unfactored tension load.
- Load in column is approx 2000 kN dead and 250kN live (construction loading)
- I will probably use two shear keys stacked on top of each other to achieve safe bearing stress on to concrete which is driving design at the moment.

To ease peoples fears, my line manager has tasked me with producing a draft design for this, then he will check it thoroughly and is a very experienced engineer. I will not be designing this solely off of the back of advice on the forum but it will help me to understand the problem and thus present a sensible, worked solution to my manager for review.

Many thanks in advance.

Attachment link:

http://files.engineering.com/getfil...c24ea7b8&file=Eng-tips_shear_key_25.11.17.pdf

 

[dik]

Thanks... Dik

 

[CANPRO]

On the question of core drilling and losing column area - your current proposal would remove 2 x 2"x25" = 100 sq.in. at a particular cross-section. So core drilling in the 20" direction of the column, you could drill 2 - 2.5"Ø holes and install 2" thru-bolts while still dealing with the same cross-section loss as the current proposal. As mentioned above, you could phase in the installation of the thru-bolts to limit the total loss of column at any given time.

I'm not a fan of the key way for loads this large, but I have to admit that its a slick way of tying into an existing column. Would you be able to share any details of the condition this was used before? Approximate loads? Maybe a picture of the finished installation? I would love to see it if you're able to share.

 

[dik]

Loss of x-section not likely an issue when you embed steel rod and grout it solid...

Dik

 

[Temporaryworks]

Ingenuity, it is interesting to know it has been done before here with the grouted pin. Were the loads of a similar order?

I do not know where my boss squirrelled away the pic of the previous project, but if I find it will take a photo and post it. I will definitely make a point to post pics of the upcoming install on this thread with a summary as a small thanks for all of your answers. Works are programmed for February 2018.

 

[Temporaryworks]

Hi all,

Attached is the part installed shear key.

Once the full truss install is complete and everything is a success I will send more pics.

Thanks,

 

[UKJim]

Have you investigated a post-tensioned concrete transfer girder? I like the concept of intentionally roughened column surfaces combined with large, permanent P/T force to transfer the shear.

 

[Temporaryworks]

UKJim, in this instance our client, the demolition contractor, had a bunch of UC's that they wanted to use up and so it was always going to be a truss.

I don't think they had the skill set to pull a post-tension girder off.

Thanks for the reply

 

[CANPRO]

TW, thanks for the followup pictures. Looking forward to seeing a picture of the finished truss.

 

[Ingenuity]

Thanks Temporaryworks for the follow up.

I am curious - your horizontal all-thread rods (with a few nuts missing) - what grade is it?

Look forward to seeing the complete truss installed and column demo'ed.

 

[Temporaryworks]

Ingenuity - it is a 1030 grade Macalloy bar.

The photo was taken during the install, we will definitely have nuts and lock nuts installed and double checked prior to removing the column.

 

[Ingenuity]

Thanks Temporaryworks.

I was hoping it was Macalloy, or equal, PT bar.

Photos after the column demo showing the elevation of the transfer truss would be good to see.