Prediction of Corner Columns’ Load Capacity Using Composite Material Analogy

-There are numerous reasons for which concrete has become the most widely used construction material in buildings, one of them being its availability in different types, such as fiber-reinforced, lightweight, high strength, conventional and self-compacting concrete. This advantage is specially realized in high-rise building construction, where common construction practice is to use concretes of different types or strength classes in slabs and columns. Columns in such structures are generally made from concrete which is higher in compressive strength than the one used in floors or slabs. This raises issue of selection of concrete strength that should be used for estimating column capacity. Current paper tries to address this issue by testing nine (09) sandwich column specimens under axial loading. The floor concrete portion of the sandwich column was made of normal strength concrete, whereas column portions from comparatively higher strength concrete. Test results show that aspect ratio (h/b) influences the effective concrete strength of such columns. A previously adopted methodology of composite material analogy with some modifications has been found to predict well the capacity of columns where variation in floor and concrete strength is significant.


INTRODUCTION
There are many reasons for which concrete has become the most widely used building construction material.Concrete's ability to shape into any structural form, easy accessibility of its constituent materials and liberty of selecting among its various types, for example, fiber-reinforced, lightweight, high strength, conventional and self-compacting are few worth mentioning reasons.This choice of selection of different grades/types of concrete favors its application also in high-rise building construction.Concrete compressive strength used in construction has been increasing over the years and strengths up to 20ksi (138MPa) and more have been used in the industry, especially in columns of high-rise buildings [1].The use of high strength concrete column sections along the height, with higher-strength concrete placed in lower stories, results in additional savings associated with repetitive use of formwork.Compared to columns, high strength concrete is not required in floor/slab region of a framed structure.Also, economy and space requirements in high-rise building construction force designers to select concretes of different types/strength classes for slabs and columns.In such a state, presence of two different grades of concrete in slab-column region raises issue of selection of concrete strength ' c f to be used in (2) for estimating column capacity.ACI-318 [2] addresses the issue of variation in strengths of column and floor concretes in its section 10.12, where it recommends no special measures as long as the ratio of column to floor concrete strength ' ' Requirements of ACI are based on [3] and state: When The maximum concentric load carrying capacity of the column can be obtained by adding the contribution of the Rearranging (2), effective strength of concrete ' ce f can be defined as ( ) In [3,[5][6][7][8][9][10], researchers tried to address this subject and proposed different expressions and solutions.Few of these studies are discussed here to justify the proposed solution presented later in the paper.
Author in [5] tested six specimens with aspect ratio h/b of 0.7 to understand the load transfer mechanism of high strength concrete column through a layer of lower strength slab concrete and to determine the effects of confinement on behavior of slab concrete.Treating the specimens as composite materials, he used mechanics of materials approach for developing (3) to calculate the effective concrete strength.
where, 0.9,1.0,1.25 G l = for corner, edge & interior columns respectively.Authors in [11] proposed (4) for computing effective concrete strength of a sandwich column and concluded that current ACI provisions for ' ( ) where 1 0.4 2.66 Authors in [7] investigated the effects of aspect ratio h/band column rectangularity on the effective concrete strength ' ce f of high strength concrete corner columns intersected by weaker slabs.The aspect ratio varied from 0.3 to 1.14 with maximum 12600psi (87MPa) concrete strength.They concluded that it would be inaccurate to not consider aspect ratio in estimating the effective compressive strength of joint.Using mechanics of material approach like [5], authors in [12] suggest (5), a design expression for prediction of interior column capacity.They are of the opinion that the composite material analogy can be effectively applied for the theoretical analysis of the problem associated with estimating column capacity.However, like [5], their suggested equation also does not acknowledge the effect of aspect ratio.
where 1.07 All mentioned studies signify the importance of aspect ratio h/b and composite material analogy approach.Expressions proposed by some researchers are based on this approach but are independent of aspect ratio, whereas expression proposed in [11] is based on regression analysis rather mechanics of material approach.

II. EXPERIMENTALPROGRAM AND TEST SETUP
The experimental program included the testing of nine sandwiched column specimens in direct compression, as these specimens adequately model the corner column slab joint [5,7].Specimens were divided into three groups A, B and C, having three specimens in each group, each group had different ratio of column to floor concrete strength.Specimens in each group had slab/floor layer of 4, 6 and 8 inches (102, 152 and 203mm).Slab portions were sandwiched between two column ends made up of comparatively lower strength concrete than the one used in column ends.Specimen with 4 inch (102mm) thick slab layer had aspect ratio h/b of 0.67 -typical to that of flat plate floor system, whereas those with 6 inch (152mm) and 8 inch (203mm) thick layer developed an aspect ratio of 1 and 1.33 respectively.Figure 1 shows the rest of the features of specimens in all groups.All specimens were tested in axial compression in a 2000kN capacity compression testing machine.First, the test specimens were installed in the testing machine, centered carefully to avoid any flexural stresses resulting from accidental eccentricity.Strain gauges applied to main reinforcement in slab region to monitor their performance under load were connected to the data logger.Three cylinders cast from the column concrete and slab concrete batches were tested first to obtain the axial compressive strength.This was followed by compressive testing of sandwich column specimens.Before testing, elastic pads were placed between specimen and machine loading plates to avoid damage to column ends.The load was applied in increments of 50kN.During the test, specimen behavior was carefully monitored, cracks were marked on their appearance along with load readings.Strain gauge readings were also recorded after each load increment.After specimen failure, the crushed concrete around the failure area was removed to observe the behavior of the reinforcement.A typical test setup is shown in Figure 2 edge and corner columns.Their results were based on tests conducted on 54 sandwich column specimens.
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TABLE II .
(7)IO OF APPARENT TO EFFECTIVE CONCRETE STRENGTH Specimens of all series confirm that the effective strength of an axially loaded column intervened by lower strength concrete floor is influenced by its aspect ratio h/b.As the aspect ratio increases, the effective strength of the joint decreases.TheACI318, Section 10.12, provisions for ' Mechanics of composite materials can be used to predict the response of slab-column joints to axial loads.The proposed expression(7)can safely be used for predicting the effective concrete strength of axially loaded corner columns. 