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Understanding Inclined Columns


Introduction


A vertical structural member subjected to axial compression in the majority with some or no moment is termed as a column. If the same member is inclined, it is a strut. But all these members in common are subjected to compression in the majority and they are categorized as strut members. 

Fig.1. Inclined columns with Architectural Importance

Sophisticated design in the construction industry is rapidly increasing due to the developments and demand of the population. One of the main structural member of all time too had gone through different changes in constructing to have a unique aesthetic view. The new idea mentioned is that columns are no longer vertical. It can be inclined. Inclined columns are originated from the category of structural framing members gravity load loaded columns and can be applied to both rigid as well as braced frames. 
Fig.2.Inclined Column Construction


When we consider an inclined column (say strut) the forces it is subjected to are axial compression, moments and shear, which can be found out by any method of frame analysis. We have mostly seen in many of the structural analysis problems we have dealt with inclined columns. But there is no variation in the method of analysis that we perform on it. The design in most of the cases is carried out for axial force and moments by the usual method . In a condition where shear is found predominant (this is the case when the member is inclined appreciably), shear reinforcement should be designed and their provision is made.

Some of the examples of inclined column constructions are rafter & struts of an RCC truss, gable beams of a sloped roof, and the top chord of an RCC Virendeel Girder.

The top chord (beam) of a Virendeel girder, which is a horizontal member, is designed as a column, and not as a beam, as compression is dominant in it.
Fig. 3.An Inclined Column Aligned in X, Y and Z direction

Load Transferring and Deflection


Fig.4. Tilted column subjected to eccentric axial load

A column that possesses a cross-section without symmetry may be subjected to torsional buckling or lateral buckling. Torsional buckling is a sudden twisting of the column. As the theories say the existence of eccentric loading would decrease in column strength. The term eccentrically loaded is defined as the situation when the axial load on the column is not concentric or in other words if the line of action of the axial load is not parallel to the central axis of the column. The eccentricity mainly represented by ‘e’, of the load subjects to bending of the column immediately. Hence the combined action of axial and bending would result in reduced load carrying ability.

Load and bending moment in an inclined column


Figure 5 below shows an inclined column connected a beam. We will discuss how we can determine the load in the column.
Figure.5

From the figure the

w = total load applied to the beam. This is the load that has to be transferred to the inclined column AB.

The axial force P can be obtained by the equation

P = wl/2

The moment due to eccentricity in the column can be given by

M = P.e

Where e is the eccentricity. From the figure, e has to be determined from the given details.

Thus 
e = (L/2) cosθ

Once the column is analyzed and the loads, forces are determined the next procedure is design


Design of Inclined column


The column design is like vertical columns
  • · Determine the type of column by calculating the effective length and the end conditions
  • · For the maximum moment, value determines the reinforcement
Many studies have shown that the inclined columns are subjected to greater moments due to eccentricity compared to vertical columns. The moment will increase with the increase in eccentricity. As the inclination of the columns is higher, the eccentricity also increases, thus the moment. The inclination in columns will result in horizontal component forces in addition to external lateral load present.



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