Concrete Pier Modification

Question

Concrete Pier Modification

bags1235 (Structural)

(OP)

14 Mar 12 16:29

Working on a bridge project in which we are modifying a three bay concrete pier bent. Existing bridge was built in 1962. Existing plans do not specify rebar grade or concrete strength. If I use the assumed material properties from AASHTO Manual for Condition Assessment when properties are unknown, the modified bent fails strength requirements. Does anyone have a AASHO reference from the 50's/60's that better states the material properties common for that period?

BridgeEI (Structural) · Answer 1 · 2012-03-14 19:23:12

I don't have any references, but are there unit stresses listed on the plans? Or maybe you could look at the pay items and at least tell you what class of concrete was used. But you'd probably also need the spec from then also.

bridgebuster (Civil) · Answer 2 · 2012-03-16 14:00:23

I have some AASHTO's from that era. I'll see what's in them.

Is your pier also failing in shear. That's not unusual due to changes in the code.

bags1235 (Structural) · Answer 3 · 2012-03-16 14:35:04

bridgebuster,
Much appreciated. Having some significant issues getting the positive moment to work with the assumed 40ksi rebar. Our agencies wants us to evaluate this 50 year old structure using LRFR. Still wondering how we could get this to work with the very different live load models of the today. Will look at shear after the moment resolves itself. So your thinking that shear will be an issue as well? Thanks in advance.

bridgebuster (Civil) · Answer 4 · 2012-03-16 18:28:16

bags1235,

From the 1957 and 1961 AASHO - Only ASD was allowed.

Flexure – Allowable stress

fc = 0.4f'c – extreme fiber in compression

None – extreme fiber in tension

Shear

Beams without web reinforcement:
Longitudinal bars not anchored or plain concrete footings = 0.02 f'c (75 psi max)
Longitudinal bars anchored 0.03 f'c (90 psi max)

Beams with web reinforcement V=0.075 f'cbjd

Steel reinforcement

Tension In flexural members or tension in web reinforcement 18 ksi – structural grade; 20 ksi – hard rail steel grade.

Compression 13.2ksi – structural grade; 16 ksi – hard rail steel grade.

Bond straight or hooked bars; except top bars .1f'c (350 max)

Top bars 0.06 f'c (210 max)

In the 1961 AASHO bond is reduced by 1/3 for bars greater than 11S

AASHO concrete classes/28 DAY STRENGTH
A or A(AE)     3 KSI
B or B(AE)    2.2
C or C(AE)    1.5
X or X(AE)    3
Y or Y(AE)    3

Column design refers back to the 1951 ACI code. If you need any more info let me know.

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Concrete Pier Modification