Development and hooks
Development and hooks
(OP)
Rather than hijacking the other thread on this I have another hook/ development question. At the top of an external column of a concrete moment frame you get a top bar that needs anchorage into the column. I am taking the standard hook and anchoring that into the outside of the column. Then I take the standard hook from the outside column bar and anchor that up into the joint. But really this is a splice of the outside column steel and the top beam reinf. So presumably the bars need to be spliced. How does a bend influence the splice length? I have also been comparing codes on this issue and it would seem that ACI gives you very small lengths when you use standard hooks but quite large lengths for development whereas the British Standard doesn't give you a great deal of advantage for a hook in terms of anchorage but the general case for anchorage gives you a smaller length than ACI






RE: Development and hooks
Then the beam top bar would extend over the support and hook into the "joint area" over the column, past the inside face of the column.
This is what I've typically seen.
RE: Development and hooks
If you need to gain more than 50% end development in tension, you have to change the detail to a larger radius bend (so you lose the extra development from the hook) and use normal development length equations. You cannot get more than 50% with the standard hook radius as it creates too much compression in the concrete in the bend, so a larger radius is needed.
So, for your detail, you should not use a standard hook, but a larger radius bend for the bars from the column into the beam and then lap the beam bars to the column bars.
RE: Development and hooks
Is this 50% from the ACI code?
RE: Development and hooks
RE: Development and hooks
You don't need to splice horizontal beam bars with vertical column bars. The bars are developed into the joint zone as described above.
DaveAtkins
RE: Development and hooks
Where a bar ends in a standard hook, the tensile development length of that end of the bar, measured from the outside of the hook, shall be taken as 0.5Lsyt.
RE: Development and hooks
you say you just need to anchor the bars into the joint zone, but what does that joint zone do with the force. Surely a top corner of a moment frame is just a bent beam with a large moment at that corner. Therefore surely the reinforcement has to be continuous over that corner.
RE: Development and hooks
I think you have a point - but I've never seen anyone do anything else with it in the hundreds of project designs I've either done or reviewed.
What you are describing is sort of a block of concrete with moment applied to two different faces adjacent to one another causing tension in the upper/outer corner. I'll have to think on that one.
RE: Development and hooks
Exactly. Try to draw the strut and tie model and you will see that you get a compression strut going from the outer corner to the inner corner. This means that effectively the tension reinforcement has to be developed beyond that outer corner node which is more than just what you have left of half the bend and the straight end of a hook.
On the other hand if it has always been done as you say and there have been no major problems - why?
RE: Development and hooks
One aspect may be that there is less stiffness in a top column and thus less moment developed there.
RE: Development and hooks
But to ACI code, 2 hooks from different directions (eg from outside column bar and top beam bar) about a transverse corner bar do not give you 100% development of the bars around that corner.
And that reinforcement has to have full capacity around the corner as I think phuduhudu is suggesting.
According to ACI a bar is only fully developed at a distance ldh from the end of the hook. The development at the corner is nominal.
RE: Development and hooks
Title: 352R-02: Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures
Author: ACI
Year of Publication: 2002
Number of Pages: 37
This document is: Active
Nonmember Price: $100.50
ACI Member Price: $56.00
Order Code: 35202
Abstract: Recommendations are given for member proportions, confinement of the column core in the joint region, control of joint shear stress, ratio of column-to-beam flexural strength at the connection, development of reinforcing bars, and details of columns and beams framing into the joint. Normal type is used for recommendations. Commentary is provided in italics to amplify the recommendations an identify available reference material.
The recommendations are based on laboratory testing and field studies and provide a state-of-the-art summary of current information. Areas needing research are identified. Design examples are presented to illustrate the use of the design recommendations.
RE: Development and hooks
that sounds like exactly what is required for this one.