Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Shear friction, or something else? 1

Status
Not open for further replies.
unners

unners

Marine/Ocean
Oct 8, 2014
8
Long time reader, first time poster :)

The company I work for is checking a steel frame that has been retrofitted to an existing concrete I-beam such that the steel strut of the frame transfers load into the chamfer of the bottom flange of the concrete beam. Hopefully the image makes things a little clearer, just ignore the numbers if it's easier!

My problem is I'm not entirely sure what failure mechanisms I should be checking. I am concerned about a failure plane vertically downward from the load point but is this simple shear or more of an interface shear problem? There's no typical beam shear reinforcement in the region, only prestressed strand. I should note that the beam is also supported on the opposite face to the strut by another steel brace, which should prevent any torsion in the beam.

I'd really appreciate any advice in understanding the problem, or any other tips on this sort of unusual loading!

Cheers

Dave

 
 https://files.engineering.com/getfile.aspx?folder=744f0909-8373-4aba-a2d8-b0f43a7391f5&file=beamsshear.PNG
Replies continue below

Recommended for you

Sort by date Sort by votes
Dave, welcome. A necessary dimension is missing, the "length" (dimension parallel to the beam's longitudinal axis) of the contact area between the frame and the beam. Using that dimension, the (minimum) crossectional area of the "shear interface" can be calculated.

I would start with a basic calc, shear on unreinforced concrete with the computed crossectional area:

1) If the results pass, you are done.

2) If the results fail by a small amount, perhaps a more accurate assumption for the contact area (distributed along some of the beam) will be a solution.

3) If the results fail miserably, look into modifying the frame to effectively lengthen the contact area as needed.

[idea]
 
Upvote 0 Downvote
That's a bit of a complex problem really. In the absence of more directly relevant guidance, I'd start by treating this as a ledge according to the method provided in the PCI handbook and discussed in this article: Link. You'll still have to get a bit creative in order to apply this to your situation but I feel that it's a good place to start. At a minimum, it'll get you thinking about all of the relevant failure modes. I do think that your shear friction check is valid but I doubt that it will govern. Rather, I suspect that the shear failure mode will look more like the three sided, diagonal tension, punching shear failure discussed in the paper.

c01_abizk9.jpg
 
Upvote 0 Downvote
I am getting lost here, where is the steel beam/strut, and where is the force/load came from? Why is only shear in concern, no torsion, nor bending , on the prestressed concrete beam? Need more pictorial detail to make your concern clear. But looks like KtootK got an edge of the puzzle.
 
Upvote 0 Downvote
Thanks very much for the welcome and the very helpful advice.

SRE and retired13, I've attached another image with more detail that may clear up a few things I've left out. The section is through the support though where the localised failure is obviously not an issue so just ignore the pile cap! The beam spans about 5m between supports, and can handle the loads globally, it's the localised failure I'm concerned with.

KootK, thanks for the link. It's exactly the sort of information I was looking for. I have had a go at a punching shear type check which I expect is somewhat similar to the ledge scenario but I'll go through that in more detail.

So far, the checks I've looked at show it's ok, but only just which is why I'd like to have a bit more confidence in my approach!

Thanks again for the help
 
 https://files.engineering.com/getfile.aspx?folder=fd9f1829-6ec6-45fe-9c93-f4bfdbf5c491&file=framesection.PNG
Upvote 0 Downvote
I think under this setup, the steel strut/brace is more prone to damage than the concrete beam. However, unlike the ledge support, the concrete flange suffers direct shear (assuming vertical as your original sketch), and bending stress from bearing on hard surface, thus the allowable stress of the concrete will be some what lower than pure shear loading. I suggest use ACI provisions for structural plain concrete to verify the strength.
 
Upvote 0 Downvote
Are you concerned about the tension in the concrete beam web caused by the 100x8 SHS on the left and BR-1 on the right both pushing down? The stirrups don't extend to the bottom of the beam, so the concrete would be carrying that tension by itself.

BA
 
Upvote 0 Downvote
I would not drill into a prestressed concrete beam, especially a heavily loaded beam, and, for sure, not where the strands are located.

The rods, nuts, and washers are 316 stainless steel... I assume for corrosion resistance (perhaps, for a coastal environment). However the SHS sections are "thin" (8mm) carbon steel.

The fasteners appear to be marginally sized for the load and there is no redundancy; one faster for each connection.

In summary, as a former bridge contractor, I would not use this overall arrangement for temporary shoring and certainly not as a permanent installation.

[idea]
 
Upvote 0 Downvote
All very valid points SRE. Yes, it's a coastal (severe) environment, the SHS are zinc painted and there are two fasteners per connection. The embedment has been specified to be well short of the strand and checked using a GPR first. (We don't like it either to be honest!) It's a semi-permanent installation so durability is of less concern than usual.

BARetired, the way I understand it is both struts (the left and right) are in compression only as the right strut is not supporting any load? Regardless, we have checked the 'tension' in the beam web through a punching shear type check.

Cheers
 
Upvote 0 Downvote
There's more to BA's direct tension check than just the punching. What you may lack here is the hanger steel required in the PCI method that I referenced. You need a way to drag the vertical load component up into the body of the cross section. I was hoping that there was more to the stirrups than those diagonal bars in the original sketch.
 
Upvote 0 Downvote
We were hoping that too KootK!

I've found some earlier papers on spandrel ledges by Klein and Raths that have some good background on hanger reinforcement/direct tension so I'll start there. Thanks!
 
Upvote 0 Downvote
Unners:
RE: Beam “B1”, in your second sketch: Why not weld a stl. pl. to the top of the beam “B1”. every so many feet, or a large stl. angle to the rt. side of the beam, with the horiz. leg going in over the top of the precast conc. beam? Saw cut slots into the edge of the existing slab for these primary load tabs in over the top of the precast bm. This puts the primary loading where it can probably be better tolerated. You will still need some bracing down to the bot. flg. of the precast conc. beam, and some bolts to transfer lateral and torsional loading, but these loads will be lighter and easier to manage down around all the tension strands, and onto the sloped top of the bot. flg., with its conc. tension and shear issues.
 
Upvote 0 Downvote
unners - This project has the characteristics of a dock or pier, not a bridge. Can you consider an alternate concept, that does not modify the existing beams / deck or put eccentric loading on the existing piling?

[idea]
 
Upvote 0 Downvote
I believe the OP is attempting to review a retrofit which has already been installed. An alternate concept may be preferable but the owner would likely not want to pursue it if the existing work is deemed adequate.

BA
 
Upvote 0 Downvote
The setup should be fine if occurs at the pile cap only. Otherwise, the left side struts will cause the girder to rotate, that is to be resisted/stabilized by the right side strut.
 
Upvote 0 Downvote
Thanks BAretired, that's the case here. It's a dock and we've been asked to review the load capacity during the installation. Thanks to all for your thoughts and advice
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

StrEng007
  • Locked
  • Question
Shear Friction at Monolithic Concrete Confusion
2
Replies
22
Views
422
KootK
KootK
VT17
  • Locked
  • Question
Shear Friction - Area of Concrete Resisting Shear Transfer
Replies
10
Views
431
SteamedHams
SteamedHams
FOX89
  • Locked
  • Question
API Tanks - seismic base shear and friction
Replies
4
Views
226
TLHS
TLHS
StrEng007
  • Locked
  • Question
Concrete frame: cantilever columns or shear walls/piers
Replies
5
Views
233
hardbutmild
hardbutmild
EngDM
  • Question
Punching shear on S.O.G. 3
Replies
11
Views
5K
lexpatrie
lexpatrie

Part and Inventory Search

Sponsor

Back
Top Bottom