SEISMIC RESPONSE OF STEEL BEAMS COUPLING CONCRETE

WALLS.

Presented by: Yahya Ali

Authors: Kent A. Harries, Denis Mitchell, Member, ASCE, William D. Cook and Richard G. Redwood.

Ductile coupled flexural walls are primary seismic load resisting systems.

Their coupling beams must exhibit excellent ductility & energy absorption.

Structural steel members with their ends embedded in concrete have been used to provide connections for manyyears in precast and cast-in-place construction.

In the following presentation we will expose the experimental results for two specimen of RC flexural walls, coupled with steel link beams and subjected to reversed cyclic-loading test.

Design and detailing of the system is also presented.

The experimented system has the following potential advantages:

1. Excellent ductility and energy absorption if the beam is properly detailed.

2. Elimination of a considerable amount of on-site labor due to the beam prefabrication.

3. Simplification of formwork

DESIGN OF STEEL LINK BEAM

The beams for specimens 1 and 2 are designed in accordancewith the seismic design requirements for EBFs in the Canadian steel design standard (Handbook 1989).

The design and detailing requirements are based on those of recommended by the Structural Engineers Association of California (Recommended 1988).

The coupling beam is the critical element it must respond in a ductile manner and exhibit significant energy absorbing characteristics.

Steel link beams are more ductile than RC beams and are able to dissipate greater amounts of energy if they are designed and detailed to yield in shear.

The shear-to-moment ratio is large for short-span link beams then:

Ultimate shear capacity will be reached before the flexural capacity.

“Shear critical" design is frequently called for.

The maximum amount of energy will be dissipated when the link beam begins to exhibit plastic behavior.

In addition, the link beams were detailed to avoid local buckling of both the web and flange and lateral buckling of the link beam.

DESIGN OF REINFORCED CONCRETE EMBEDMENT REGIONS

At the faces of the walls, the link beam transmits a shear V and a moment of Vl/2 to the reinforced concrete embedded region.

Shear and moment diagram of the beam:

Where the nominal shear capacity Vc of the concrete embedment is given by:

The embedment length le was chosen such that the vertical load capacity of the concrete embedment Vc exceeds the plastic shear capacity of the link beam Vp which is equal to 1.25xVn

Vn: the nominal shear strength

Where:e = eccentricity of resultant shear loads from the center of the embedment (i.e., e = l/2 + le/2); f’c = compressive strength of concrete.

It is therefore necessary to provide sufficient vertical reinforcing bars crossing the link beam-concrete interface.

Under loading, a gap is created at the link beam flange-concrete interface.

A larger proportion of reinforcement is concentrated near the face of the wall.

Details of link beams of specimen 1 and 2:

Specimen 1:

Specimen 2:

Material properties:

Details of reinforcing cage of both specimen:

Details of reinforcing cage and link beam of specimen1:`

Linear voltage differential transformers (LVDTs) recorded vertical displacements of both the link beams and the concrete walls.

Electrical-resistance strain gages recorded the strains in the link beam, wall reinforcing bars, and on the face of the concrete walls.

Load cells recorded the applied forces.

Both steel link beam specimens exhibited great energy absorption.

Specimen 2 exhibited the largest levels of hysteretic damping, with the least amount of decay with cycling.

This damping coefficient β is defined as:

Where:A1 = area within the hysteresis loop of one half cycle

A2=area of the triangle defined by an equivalent elastic stiffness to the peak load of each half cycle.

The decay of energy absorption occurring with cycling at each ductility level is the width of the response band.

Experiment results:

Link Beams of Specimen 1 (a) and 2 (b) after removal from Walls

Equivalent Elastic Damping Coefficients for Specimens I and 2 and Reinforced Concrete Coupling Beams

Specimens 1 and 2 after testing:

Specimen 1

Specimen 2

DESIGN STEPS OF LINK BEAM:Step 1Determine the web dimensions to resist the required shear strength Vu from:Step 2Determine the required moment resistance φbMn:

Step 3Choose a beam section that satisfies the width-thickness ratios of Table 8.1 of AISC (Seismic 1992). Provide full-depth intermediate stiffeners with details and spacing according to clause 10.3 of AISC (Seismic 1992).

Step 4To control out of plane buckling, the maximum unsupported length Lp is given as:

Step 5Provide full-depth stiffeners wide enough and having a thickness not less than 0.75tw or 10 mm.

Provide an additional full-depth intermediate stiffener on the embedded region of the web at a distance equal to the cover inside the face of the wall.Either provide additional intermediate stiffeners to obtain shear resistance >= Vp in clear span or increase the web thickness in the embedment such that:

Step 6Determine the required length of effective embedment leff such that:

Step 7Choose the area of vertical reinforcing bars As, such that:

CONCLUSIONS:The results of this experimental program have shown that:

It is possible to achieve excellent ductility and energy absorption characteristics by using this system.Additional stiffeners are provided in the region of expected cover spalling

The embedment region must be designed for a shear and moment corresponding to the development of the full capacity of the link beam.

Vertical reinforcing bars are placed near the inside face of the walls

The use of this system provides excellent quality and greatly simplifies the formwork and steel placement.The use of steel link beams in RC walls is an alternative to traditionally or diagonally RC coupling beams.The excellent performance of this structural system is well suited for structures designed for high levels of ductility.

THANK YOU