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BEAM HOOP REINFORCEMENT FOR LARGE CONCRETE SMRF BEAMS

PRELIMINARY TEST RESULTS FOR BEAM 1

A project funded by the Pankow Foundation, Webcor Builders, ACI Foundation's Concrete

Research Council, and the CRSI Foundation

By Grigorios Antonellis, Tea Visnjic, Marios Panagiotou, and Jack Moehle

University of California, Berkeley

25 January 2011

BACKGROUND

Recent developments in the construction of high-rise buildings in the seismically active West

Coast have resulted in construction of reinforced concrete special moment frame beams larger

than was typical in past practices. The current Building Code requirements for these beams were

written around prevailing practices from many years ago and, when applied to these new

buildings, can result in hoop spacing as large as 12 inches in the beam plastic hinge zone (see

ACI 318-08, Section 21.5.3.2). Some engineers have questioned the performance capability of

these beams and have recommended Building Code changes to reduce the maximum hoop

spacing. An ongoing research project at the University of California, Berkeley is investigating

the requirements through a laboratory test program.

TEST SPECIMENS (Figures 1 through 4)

Two test beams have been designed and constructed to test current Building Code provisions and

some proposed Code changes. The beams cantilever from a common reaction block that is

anchored to the laboratory floor (Figures 1 and 2). The beams are tested by imposing

displacement cycles (upward and downward) to simulate the effects of deformation reversals that

occur during a major earthquake.

Beam 1 (Figure 4) is designed to satisfy all provisions for special moment frame beams

according to the current Building Code provisions. The beam hoops (No. 5 at spacing s = 11

inches) are each made up of three pieces; a stirrup with seismic hooks, a crosstie at the top to

close the hoop, and an additional vertical crosstie to restrain longitudinal bars along the top and

bottom faces. Volumetric confinement reinforcement ratio is ρs = 0.31%, where ρs = volume of

vertical legs of hoops divided by volume of core. In Beam 2, the hoop spacing s is reduced to 6

inches, corresponding to a volumetric reinforcement ratio ρs = 0.57%. Concrete is normal weight,

using pea gravel (not crushed) as the maximum size aggregate, with target compressive strength

of 5000 psi. All reinforcement is Grade 60.

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Figure 1. Plan view of specimen - test setup of Beam 1.

Figure 2. Side view of specimen – test setup of Beam 1.

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Figure 3. Side view of specimen showing anchorage of beam longitudinal reinforcement.

Figure 4. Cross sections of Beams 1 and 2.

AS BUILT INSTRUMENTATION

The instrumentation of the test specimens consists of strain gauges attached on the reinforcing

steel and linear potentiometers connecting between steel rods that were anchored into the

concrete core. Drawings of the instrumentation plan can be found in the appendix at the end of

this report.

BEAM 1 TEST RESPONSE OVERVIEW

This section presents some preliminary observations from the test of Beam 1 (designed

according to the current ACI provisions, with 5 #11 longitudinal bars at top and bottom and #5

hoops at 11” spacing). Force-displacement diagrams are shown in Figures 5 and 6. Figure 5

shows the response up to the peak drift ratio of 5.8% reached in this test. Figure 6 shows the

response for drift ratio up to 3%, where first buckling of the longitudinal reinforcement was

observed. Positive displacement and force are for loading the beam tip downward.

IMPORTANT OBSERVATIONS

The first buckling of the longitudinal bars occurred during the first cycle of 3% drift ratio, after

reaching the peak drift ratio. All the top longitudinal bars buckled together creating a “crack

cave” that remained open for all the remaining cycles (Figures 7, 8 and 9). During the second 3%

drift ratio cycle the strength of the beam reduced by 30%. For the next two cycles with amplitude

3.8% and 5.5% the resistance of the beam reduced to 50% of the peak response.

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Another important note is that the beam showed signs of impending shear failure prior to

buckling of the longitudinal reinforcement. After buckling occurred, shearing deformations

contributed significantly to the total beam displacement and were especially notable for

downward displacement (compare Figures 10 and 11).

Figure 5. Force-Drift ratio diagram (all cycles).

-200

-150

-100

-50

0

50

100

150

200

-6 -4 -2 0 2 4 6

FOR

CE

(kip

s)

DRIFT RATIO (%)

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Figure 6. Force-Drift ratio diagram (only cycles up to and including 3% drift ratio).

Figure 7. Initiation of buckling of the top reinforcement (first cycle with peak drift ratio = 3%,

instantaneous drift ratio = -0.9%).

-200

-150

-100

-50

0

50

100

150

200

-3 -2 -1 0 1 2 3

Forc

e (

kip

s)

Drift Ratio (%)

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Figure 8. Buckling of the top longitudinal bars (first cycle with peak drift ratio = 3%, instantaneous drift

ratio = -3%).

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Figure 9. Overview of the beam after the buckling of the top longitudinal bars (first cycle with peak drift

ratio = 3%, instantaneous drift ratio = -3%).

Figure 10. Overview of the beam at the maximum positive displacement (cycle with peak drift ratio =

5.8%, instantaneous drift ratio = 5.8%).

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Figure 11. Overview of the beam at the maximum negative displacement (cycle with peak drift ratio= -

5.8%, instantaneous drift ratio= -5.8%).

APPENDIX

STRAIN GAUGES

Figure 12. Location of instrumented transverse reinforcement.

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Figure 13. Location of instrumented longitudinal bars in Beam 1.

Figure 14. Strain gauges layout of longitudinal bars - type A (10 strain gauges).

Figure 15. Strain gauges layout of longitudinal bars - type B (7 strain gauges).

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Figure 16. Strain gauges layout of stirrups - type C (4 strain gauges).

Figure 17. Strain gauges layout of stirrups - type D (2 strain gauges).

LINEAR POTENTIOMETERS

Figure 18. Plan view showing location of linear potentiometers attached along the top face of Beam 1.

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Figure 19. View from underside showing location of linear potentiometers attached along the bottom face

of Beam 1.

Figure 20. Side view showing location linear potentiometers attached along the side face of Beam 1.