SEISMIC PERFORMANCE OF SLAB SHEAR REINFORCEMENT

Flat slab building systems are utilized extensively for construction of apartments, hotels and office buildings. Flat slab construction is common throughout the USA and indeed much of the World. Here in Hawaii, the large visitor and tourist trade has resulted in the construction of a large number of hotels and rental apartment buildings. The majority of these low to mid-rise buildings utilize flat slab construction.

The advantages of a flat-slab floor system are numerous. The simplicity of formwork and resulting speed of construction makes for a short and economical construction period. Low floor to floor heights (generally 2.5 to 3 meters) reduce the total building height, thus reducing lateral loads, cost of building cladding, cost of vertical mechanical and electrical lines, and air conditioning/heating costs.

The economy of this type of construction is further enhanced by utilizing the simplest form of flat slab, namely a flat plate system with no variations in slab depth. A critical design criterion for flat plates is the punching shear strength of the slab at the slab-column connections. To avoid adding drop panels or beams to increase the shear capacity of the slab, various types of shear reinforcement can be used in the slab around the connection. Some of the more convenient types of shear reinforcement have never been evaluated under seismic loading conditions.

The proposed research program involves the fabrication and testing of four large-scale flat-plate interior slab-column connections with some of these common types of slab shear reinforcement when subjected to cyclic lateral loading.

RESEARCH OBJECTIVES

The intent of this program is to study the response of slab-column connections containing various types of shear reinforcement when subjected to cyclic lateral loading. Considerable research has been performed using different types of shear reinforcement, though most of that research has focused on static loading under gravity and lateral loads. The satisfactory performance of a particular type of shear reinforcement under static loading is not an adequate indication that it will perform satisfactorily under cyclic lateral loading conditions as will occur during an earthquake.

Various types of shear reinforcement will be considered in this study:

1. Closed hoop stirrups with a longitudinal slab bar in each corner of the stirrup. Placement of these stirrups is difficult and hence they are not preferred by the construction industry.
2. Individual stirrups enclosing a single reinforcing bar in both the top and bottom layers of reinforcement. These stirrups are considerably simpler to place than the closed hoop stirrups.
3. Shear studs in the form of round bars fully anchored at the top and bottom by steel plates. These studs are typically fabricated in stud-rails for ease of installation. They are proving more and more popular because of their ease of placement and reas onable cost.

Test Specimens

This research program will consist of testing four slab-column connections. The specimens will be a half-scale representation of an interior connection in a prototype flat plate building. All test specimens will be identical except for the type of shear reinforcement used in the slab. Four types of shear reinforcement will be considered as follows:

Specimen 1 - No Shear Reinforcement (Control specimen);
Specimen 2 - Closed hoop stirrups;
Specimen 3 - Single leg stirrups;
Specimen 4 - Headed studs.

The specimens will be subjected to an incrementally increasing cyclic lateral displacement routine. The intent of this displacement routine is to study the connection behavior under increasing levels of lateral drift and to determine the failure drift level for a connection using a particular type of shear reinforcement.

The specimens will be fully instrumented during the test to record all relevant loads, displacements and strains. The experimental work will be carried out in the structures testing laboratory in Holmes Hall. This 300 square meter laboratory is equipped with a 115 square meter strong floor. The reaction points on the strong floor are capable of resisting forces well in excess of those anticipated during the proposed test program. The laboratory is equipped with a hydraulic pump and servo-controlled MTS hydraulic actuators, controlled by an MTS TestStarII Digital Controller. All instrumentation readings will be recorded by a National Instruments Data Acquisition System connected to an IBM 486 Personal Computer.

ANTICIPATED ACCOMPLISHMENTS

The seismic performance of common and convenient open types of slab shear reinforcement will be evaluated when compared with conventional closed stirrups. Adequate performance of these types of shear reinforcement will lead to increased economy in flat slab construction in seismic zones here in Hawaii and in the rest of the USA. Inadequate performance may lead to restrictions on the use of these types of shear reinforcement in seismic zones, thus increasing construction safety.