jrs_87 said:
This thread is helping me understand the 11/12 joint.
A comment - after all, you asked. I have used the shear -friction concept for a career. No problems. But I see it as most useful in flanges to girders and such. Continuous walls - few retaining walls will have a shear of 1300 kips from a slice of soil every 1'-9" along the wall (I chose 1'-9" because that is the width of member 11 and its shear contact with the deck).
The concept of shear-friction can be used wherever you can draw a line (plane) where shear might develop basically parallel to that drawn plane. At the face of a column with a cip corbel, for instance. But in that case (cip) a factor of 1.4 is used. Intentionally roughened (1/4") a factor of 1, and as cast (even finished or formed?) the factor is 0.7. They say this is confirmed by testing. Concrete to steel is also 0.7. Using numbers like that for a condition like this truss is scary - I recommend reaching into the bucket for a lot of redundancy. With the mating of two basically smooth surfaces just how much axial strain can be developed in reinforcing steel? Large bars need a lot of development length - or half length and a 90 bend. I find it questionable that that yield strain in the reinforcing will maintain contact sufficient for a friction factor of 0.7. (That should garner me some comments). But I do not write codes nor do I conduct lab tests. And varying points of view stimulate debate and conversations, I hope.
As to the 11/12/deck joint, there was a plane that can be identified thru the failed zone almost parallel to the slope of member 11, and that pretty much describes the failure in the OSHA pics 62,62,64.
Because the #7 hoops across the deck plane sheared, they had about 80 kips to contribute until they failed, then nothing across the deck remained except some friction from the normal force (the flat joint had slipped days before the collapse). It appears the deck plane held until passing the #9 bar at the top interior face of the diaphragm, then began to slope down along the incline of 11, until it joined the 8" pipe sleeve. The width of the failing block increased until reaching the highly compressed regions of the D1 PT strands. Note on the west side (Rt in the pic) the coils that contain the bursting pressures under the PT anchors. It appears the breakout continued in a diagonal widening like in diagonal tension. And no diagonal tension reinforcing to help.
I read the divot in the cold joint at the bottom PT anchor and stub as having been pryed up as the PT ripped from the bottom of 11. If that idea has merit, the cold joint was basically flat all the way to the #9 bar, where the shearing turns down and begins widening and developing diagonal tension, eventually daylighting out the end.
Getting back to the action of an intentionally roughened cold joint, I have some difficulty with a "raked joint". I would rather see a ribbed area formed with cured surfaces if we want to assume a 45 degree slope and a subsequent development of tension in reinforcing across the plane. I question at what point in a "slide" does the raked concrete crumble and effectively lubricate the joint? It has probably already failed at that point, I suppose.
For forces in the order of those on 11, I would have investigated a formed an integral well reinforced and well anchored bulkhead/block cast with the deck, with a joint face perpendicular to the axis of 11. And added plenty of confinement reinforcing at all joints. But then I would not be doing this project.
In addition, I think there may be issues with the aggregate strength at this level of stress (8500 psi). The splitting tensile strength could be critical and might affect joints and shear-friction.
And there was mention of changing suppliers of the concrete - was there time to test the new product? (28 day test?). Was the reinforcing well cleaned or replaced, and where was the concrete removed?
Just a comment - -
Thanks.
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