International Concrete Abstracts Portal

Lateral displacement of multi-story flat plate concrete buildings in an earthquake induces moment reversals between columns and slabs. The amplitude of the transferred moment depends upon the story drift, defined as the displacement of one floor relative to the floor above or below. Flat plate buildings must have a lateral force-resisting system that limits the design story drift ratio to 0.025; where the design story drift includes plastic deformation and is defined as the design story drift divided by the distance between the mid-surfaces of the flat plates of two consecutive floors. The moments transferred from the columns to the slabs have to be resisted by flexural and shear reinforcements, whose magnitudes and detailing provide the slabs with the strength and the ductility to undergo the design story drift without failure. The design of shear reinforcement for the moment transfer in an earthquake, as required by ACI 318, considers either the strength or the ductility, not both. ACI 421.2R-10 recommends and justifies a design procedure for the shear reinforcement providing the strength as specified by ACI 318; in addition, it recommends a minimum amount and extension of shear reinforcement that provides a level of ductility adequate for a design-story drift ratio = 0.025 (the upper permissible level in several codes). The design procedure is presented with examples.

Un-bonded post-tensioned slabs were developed and principally flourished in the USA since the mid 1950’s. The continuous un-bonded one-way post-tensioned slabs became popular due to their predominant use in parking structures all over the country which is true even now. Two-way un-bonded post-tensioned slabs, mainly flat-plates and flat-slabs gained popularity since mid 1960’s. The use of a Banded System of placement of un-bonded post-tensioning tendons, introduced in the early 1970’s, made flat-plate and flat-slabs more competitive. Flat-plate/slab and shear-wall system became and remains very popular for mid-rise and hi-rise buildings. A brief discussion on the development of un-bonded post-tensioned slabs and its relevance in current design and construction is made in part-1 of this paper. Many of these slabs are now between 30 to 60 years old. These structures need routine repairing and retrofitting work. Existing methods are labor intensive and expensive. An alternative method could be repair work with composite materials. Use of composites (mainly CFRP) as a repair material for concrete structures is becoming very common. Most of the repair procedures are based on researches with reinforced concrete specimens and in some cases with pre-tensioned specimens. Research work using un-bonded post-tensioned specimens, especially two-way slabs is practically non-existent. A testing program was developed with the goal of finding the cracking and ultimate strength behavior of un-bonded post-tensioned slabs (before and after repair with CFRP) with different boundary conditions. A total of six slabs were tested. In the first phase two two-way simply supported un-bonded post-tensioned slabs were tested. In these tests CFRP repair configurations were varied. In one case CFRP was placed across the cracks in another case it was orthogonal (parallel to the edges). In the second phase, four more slabs were tested (three one-way slabs and one two-way slab). One one-way slab had two ends fixed, another one had one end fixed and one end simply supported and the third one had both ends simply supported. The CFRP placement configuration in these three slabs varied. CFRP was placed across the cracks in the supports and mid spans. The fourth slab was a two-way slab with simple support. In this case the CFRP repair configuration was similar to the first slab (but CFRP had 2 inches/5 cm overlaps). Sketches of different cracking patterns and CFRP configurations are shown inside. The repair and testing of slabs is discussed in part-2 of this paper.

This paper deals with concrete slab-column connections reinforced with shearbands, covering the performance under gravity and combined gravity and cyclic lateral loads. Prior and recent test results from the U.K., U.S. and Korea are summarized. The recent tests conducted at Seoul National University revealed that the shearbands were more effective in increasing punching shear resistance, deformability and energy dissipation than headed studs under the same testing conditions (e.g., flexural and shear reinforcing ratios, gravity shear ratio = ~0.45, etc.). Engaging only a few slab bars was sufficient for anchorage. The top and bottom bends appeared to play a significant role in providing shearband anchorage. For the cyclic lateral tests (Kang and Wallace, 2008; Park et al., 2011), the constant gravity load was maintained by continuously jacking up the column bottom during seismic testing of the slab-column connections. This was the same method used for all connections in each seismic test program. The details of all the testing programs and design oversights are discussed. Finally, practical applications of shearbands in North America and Australia are introduced.

Reinforced concrete two-way slabs have evolved in the past century from simple solid slabs supported on beams on all sides to a multitude of complex slab systems, including irregular geometries. The demand for longer spans, advancements in forming and placing systems and the requirement for architectural esthetics, resulted in the innovation of today’s diverse two-way slab systems. When deciding on a slab system for certain project, designers now have the advantage of choosing from a wide variety of economical systems to suite the project requirements. Extensive experimental and analytical studies performed in the past hundred years and field observations of existing slabs helped researchers and engineers understand the complex behavior of two-way slab systems. With the increased understanding of the behavior of slabs, methods of analysis; design provisions and Code requirements have improved with each code cycle. This paper discusses the evolution of the different two-way spanning slab structural systems and reviews the historical development of the code provisions

The structural design for the East Addition to the St. Cloud Hospital facility in central Minnesota incorporated punching shear and cracked section design criteria that are not currently specified in ACI 318-05. The complexity of the column layout and shallow floor-to-floor spacing were the primary reasons for choosing a twoway 12 in. (300 mm) reinforced concrete flat slab for the 450,000 square foot (42,000 m2) addition. The use of continuous top and bottom reinforcing mats and the use of column capitals were early design decisions. Due to the complex column layout neither the Direct Design Method, nor Effective Frame Analysis would have been ideally suited to this project. A finite element analysis based program was employed to determine required flexural reinforcing, column joint forces and slab deflections. Design methodologies were investigated, and a method from the literature was chosen that incorporates slab depth, aggregate size, and reinforcement ratio when determining punching shear resistance, resulting in a reduced punching shear capacity. Another design consideration was the use of a reduced modulus of rupture to better predict deflection performance. Construction of the primary concrete structure has been completed and no performance issues have been observed.