In reinforced concrete one-way slabs, two limit states related to shear need to be checked: beam shear over an effective width at the support and punching shear on a perimeter around the load. Current code provisions are based on shear tests on heavily reinforced slender beams under point loads. The question remains if these procedures are valid for wide beams and slabs under point loads close to the support. To evaluate the shear capacity of reinforced concrete slabs and the associated effective width, a series of experiments is carried out on eight continuous one-way slabs and twelve continuous slab strips loaded close to the simple and continuous supports. Test results are compared to current code provisions and methods to calculate the shear capacity from the literature. The influence of the shear span to depth ratio, the size of the loading plate and the overall width of the specimen are discussed. From these results follows that the behavior in shear of slabs and beams is not identical. The effective slab width, used for calculating the beam shear capacity, is recommended to be based on load spreading under 45° from the far side of the loading plate towards the support.

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.

The ductility and the strength of flat plate connections with their supporting columns are influenced by the concrete strength, the thickness of the slab and the shear and the flexural reinforcements. The present paper concentrates on the important effect of flexural reinforcement in the presence or the absence of shear reinforcement.