huangyhg

new to existing concrete beam connection
can this be done? we need to add new concrete beam to a coal unloading station. the beam will span 14' and will connect to an existing concrete beam and an existing concrete wall. the existing beam is 4' wide x 6' deep and the wall is 4' thick. the existing beam has ties at 6" oc, top and bottom bars, and face bars. the idea was to demo a beam seat into the existing beam and wall so that a bearing connection is obtained. the new beam is to support one train rail for coal unloading trains (cooper 80 loading). therefore, we didn't want to just dowel into the side of the existing beam and wall. didn't know if it was feasible to chip out a beam seat...leaving the existing reinforcing intact and placing the new concrete beam.
any better solutions using concrete? if not, might have to use steel and drop some columns.
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seems like the built-up steel girder(s) solution in two or more sections for placeablilty considerations, with steel columns, is a much better option here. professionally, i am uncomfortable with the scenario of disturbing the integrity of such a large beam with so much reinforcing. moreover, any hope of chieving the nominal moment capacity of the top bars for concrete beams seems very unlikely.
mike mccann
mccann engineering
twinnel,
i would use steel if at all feasible. any work with concrete would require a much larger outage time (read cost).
csd
any chance you can set another column - steel or concrete??
there's nothing wrong with using shear friction principles to connect the new beam to the existing beam and wall (aci 318, chapter 11.7). all you have to do is develop the yield strength of the shear friction bars by epoxying to the required depth. for instance, using hilti hit hy 150 max adhesive, required embedment is only 5 5/8" for #5, and 6 3/4" for #6.
i have actually removed hundreds of columns from the face of an existing, 6-story post-tensioned parking garage by doweling the old edge beam into the face of a new edge beam on the building expansion, and then sawcutting out every other existing column. shear friction really works!
twinell,
if you are deciding to drill and dowel in bars, you need to take caution so that the existing bars in the concrete do not get damaged.
i think i am leaning towards adding steel beams under the train rail and epoxy anchoring them into the existing concrete wall and beam. the train will be going very slow and stopping on this beam and the largest span is 14'. i will probably double the required number of anchors for redundancy....that makes it a factor of safety of 8. sounds too conservative.
i would think that is reasonable in this case.
check that the anchors are appropriate for vibrating loads, and that they are not in constant tension.
csd
i like to chip away some of the existing section to "key" any new concrete into the existing.
i believe that shear friction works but epoxy adhesives don't always work as advertised. especially in constant tension.
my preference would be to chip out the beam seat required, without disturbing the existing reinforcement. drill in some epoxy anchors just to tie the whole thing together, and cast your new beam. i can't imagine how this would have an adverse affect on a 4' wide x 6' deep beam. design your new beam as simply supported.
agree with hokie66.
- design new beam as simply supported
- use epoxy anchors, old / new concrete interface should be well roughened
- if vibration involved use hilti hvu system or other equivalent system. hilti hy 150 and re 500 don't carry approval for vibration loads
- if beam is designed as fixed at both ends; there will be tension load applied on the anchors. then the deformation of anchors under applied loading should be checked as excessive deformation might diminish the effective shear carrying capacity at the old / new concrete interface which had been roughened
clefcon

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