UNIVERSITY
OF HAWAII
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
Cyclic
Testing of Slab-Column Connections
With Slab Shear Reinforcement
James Lee and Ian Robertson
BACKGROUND INFORMATION:
Name James Lee
University of Hawaii
Department of Civil and Environmental Engineering
2540 Dole Street, Holmes Hall 383
Honolulu, HI 96822
Name Ian Robertson
University of Hawaii
Phone (808) 956-6536
Department of Civil and Environmental
Engineering
Email ianrob@hawaii.edu
2540 Dole Street, Holmes Hall 383
Honolulu, HI 96822
TEST SPECIMEN DETAILS
Specimen 1C – No shear reinforcement
Specimen 2CS – Closed hoop stirrup shear reinforcement
Specimen 3SL – Single leg stirrups shear reinforcement
Specimen 4HS – Headed Stud shear reinforcement
SUMMARY
Overview
Flat slab building systems are a familiar sight in Hawaii and most of the world today. Flat slabs are used in many apartments, hotels, and office buildings. The advantages in designing for a flat slab system are numerous. The formwork design for flat slabs is simple, which reduces construction costs. Flat slabs reduce the floor to floor heights, allowing for added floors or a reduction in the overall height of a building. Reducing the building height reduces the lateral loads, cost of mechanical and electrical lines, and heating and air conditioning costs.
Although flat slabs are an economical structural system, as with all structural systems they must be designed for certain critical events. A critical event that typically occurs in flat slab systems is punching shear. Punching shear is a mode of failure that occurs between the slab and the column without warning. Failure of one or more slab and column connections may lead to collapse of a large area within the total slab area or the entire structure.
Punching shear failure can result from excessive gravity loads or a combination of gravity loads and lateral loads due to wind or earthquake. The test program reported here investigates the performance of slab-column connections subjected to combined gravity and lateral loads. It is shown that the use of slab shear reinforcement greatly improved the connection performance.
General
Four slab-column specimens were tested in this program. Each of these specimens represents an interior flat-slab and column connection in a prototype flat plate building, as shown in Figure (1).
All four specimens were subjected to a constant gravity slab load and a cyclic lateral displacement routine. The program was designed to study the performance of various types of slab shear reinforcement when subjected to cyclic lateral loads.
Prototype Design and Specimen Design
The prototype building represents a typical office
or apartment building. The design gravity loading on each floor consisted of
the self-weight of the slab plus a 20 psf superimposed dead load and 50 psf
superimposed live load. The lateral earthquake loading was based on the NEHRP
design recommendations (Ref. 2.1) for a Category 2 or moderate earthquake (Ref.
2.2). The frame was analyzed for this type of lateral loading to find the
moments and shear forces at the slab-column connection on the second floor
level. These moments and shears were then added to those obtained from a
gravity load analysis to get the design slab moments and shears. The slabs were
then designed using the ACI 318-95 Building Code (Ref. 2.3). The final slab
thickness of 9 in. was selected such that the maximum ultimate shear on the
slab critical perimeter was equal to n c
= 
(psi
units), where
is the concrete compressive strength (in psi).
The test specimens were a half-scale replica of
the prototype connection and were designed using the same criteria used in the
design of the prototype building (see Fig. 2). During the test, the slab gravity loading
simulated the full dead load plus 30% live load on every floor. This slab
loading resulted in a direct shear stress (n
= P/A c) equal to 
around the critical perimeter of the prototype and the specimen
connections prior to the start of the test. Application of this slab loading to
the specimen was anticipated to cause flexural slab cracking prior to testing.
In actual buildings, slab-column connections are often subjected to large loads
prior to a design earthquake event. For example, construction shoring loads,
prior live loads, and previous earthquake damage are a few factors that induce
slab flexural cracking. The flexural reinforcement in each of the slabs
consisted of No. 3 deformed bars positioned in accordance with ACI 318-95 code
procedures (Ref. 2.3).
|
Table 1 |
||||||||
|
Specimen |
fc' |
fr |
d ave |
d flex |
r within c2+3h |
r outside c2+3h |
||
|
(psi) |
(psi) |
(in) |
(in) |
r top |
r bottom |
r top |
r bottom |
|
|
1C |
5128 |
651 |
3.6 |
3.94 |
0.83% |
0.36% |
0.31% |
0.31% |
|
2CS |
4552 |
514 |
3.6 |
3.94 |
0.83% |
0.36% |
0.31% |
0.31% |
|
3SL |
6296 |
629 |
3.6 |
3.94 |
0.83% |
0.36% |
0.31% |
0.31% |
|
4HS |
5543 |
506 |
3.6 |
3.94 |
0.83% |
0.36% |
0.31% |
0.31% |
Reinforcement
Each of the four slab-column specimens contained the same flexural steel reinforcing. The slab reinforcement consisted of No. 3 reinforcement bars distributed as shown in Figures 3 and 4. The steel in the slab ran in both directions and three top and three bottom reinforcement bars passed through the column. The column was reinforced with eight No. 7 vertical bars with No. 3 ties at 3 inches on center as shown in Fig. 2.4. The column was designed so as not to yield prior to slab flexural yielding. Three types of slab shear reinforcement were used as shown in Fig. 5.
Test Setup
The specimens were tested in a steel reaction frame as shown in Figure (6). The specimen was lifted by means of a 20-ton overhead crane. The test setup was designed to monitor the shear and axial reactions during the test. The top of the column was connected to a load cell to properly monitor the force applied to the specimen. The lateral loading was applied to the specimen by a hydraulic actuator. No axial load was applied to the column as column axial load does not have a significant effect on the connection behavior since connection failure invariably occurs in the slab around the column and not in the joint itself.
The bottom of the specimen rested on a load cell measuring the vertical reaction while the column base was restrained against lateral movement. The shear at the base of the column was obtained from horizontal equilibrium. Equilibrium of vertical loads and reactions, and overturning moment equilibrium were used to obtain the vertical reactions under the interior columns.
Specimen Loading
The gravity load on the slab-column specimens totaled 13 kips. Twenty ¾ inch diameter holes were created in the slab during he pouring of the concrete at the locations shown in Figure (7). Sixteen concrete blocks, approximately 14"x14"x28" and weighing 450 pounds each were cast. A steel carriage with steel rods totaling 1800 pounds was used for the four interior loads on the side of the slab above the reaction frame. All the weights were suspended from 5/8-inch diameter steel rods bolted through the holes in the slab.
The locations of the weights were chosen so that
the shear and moment induced on the specimens would be similar to the moments
felt in a continuous flat plate system. As mentioned earlier, the design
criteria for the shear capacity at the face of the column due to the gravity
loads needed to be approximately . The slab loading was applied while the load
rods were free to move. The theoretical shear and moment values for the
continuous slab due to the gravity loads at one face of the column were 6444
psi and 118.2 k-in., and the values for the test specimens were 6820 psi and
149.6 k-in., respectively
Test Routine
The predetermined cyclic lateral displacement routine was applied at the top of the column through a displacement-controlled actuator. The intent of the routine was to study the behavior of the slab-column connection while subjecting the specimen to increasing levels of lateral displacement and to determine the failure drift level for the connection using a particular type of shear reinforcement. The routine consisted of two phases. Phase I consisted of twenty-one cycles ranging from 0.25 percent to 5 percent of the total column height. Within this routine a series of repeated 1- percent drift cycles were incorporated to measure the stiffness degradation while increasing drift was imposed on the specimen. At various drift levels during each cycle, the test was paused in order to collect data and observe slab cracking.
Phase II of the predetermined cyclic routine was added if the specimens did not fail during Phase I. Due to the six-inch stroke limitation of the actuator, cyclic testing in excess of 5 percent drift was not possible. Phase II consisted of 6 half cycles (see Figure 8) with peaks ranging from 5 percent to 8 percent in the positive direction. Initially the test was to include only drifts to 5 percent. However due to the excellent behavior of the connections with shear reinforcement, the unidirectional testing was added to observe if the slab would fail at higher displacements.
FEMA 273/274 Comparison
Slab-column connection test data collected to date has been compared with FEMA 273/274 provisions. Preliminary observations indicate that the FEMA connection stiffness predictions over-estimate the actual stiffness. The researchers suggest a modified stiffness be taken into account which is based on assuming the cracked stiffness as the initial stiffness. The viewer may observe the FEMA plots for each of the specimens on their corresponding web-pages.
Test Results
The results for each of the test specimens are presented individually at the following locations:
Specimen 1C – No shear reinforcement
Specimen 2CS – Closed hoop stirrup shear reinforcement
Specimen 3SL – Single leg stirrups shear reinforcement
Specimen 4HS – Headed Stud shear reinforcement
Stiffness degradation plots were generated along with the hysteretic plots. Please refer to the web pages of the individual specimens for the downloadable EXCEL 97 version files.
A compilation of the backbone curves for all four specimens is shown in Figure 9.
Figure 3
Figure 4
Figure 9
