Analysis and Design of Columns

The study guide shall focus on the analysis and design of reinforced concrete columns. The topics include the principles/concepts of short reinforced concrete under axial and flexural loading.

WHAT IS A COLUMN?

In general, columns are classified as long or short columns. If the height of a column is less than three times its least lateral dimension, it may be considered as pedestal. Pedestals may be designed without reinforcement with a maximum permissible compressive strength of 0.85(phi)f'c, where phi is equal to 0.65.
If the compressive strength is greater than this value, the pedestal will have to be designed as a reinforced concrete short column
If the length of the column is increased, the chances it will fail by lateral buckling will be increased. Columns that fail by buckling are called long columns

types of columns

If the column has a series of closed ties, it is referred to as a tied column.
If a continuous helical spiral made from bars or heavy wire is wrapped around the longitudinal bars, the column is referred to as a spiral column. Spirals are even more effective than ties in increasing a column’s strength.
Illustrated also in the figure shown are concrete columns that are reinforced longitudinally by structural steel shapes, which may or may not be surrounded by structural steel bars, or they may consist of structural steel tubing filled with concrete.

AXIAL LOAD CAPACITY OF COLUMNS

Axial load without moment is not a practical case in the design of columns, but the discussion of such case is necessary for explaining the theory involved in eccentrically loaded columns. For a column subjected purely by an axial load, the nominal load Pn that it can carry is the sum of the strength of steel which is fyAst and the strength of concrete 0.85f'c(Ag-Ast) where Ag-Ast is the net concrete area, or

AXIALLY LOADED SHORT TIED COLUMNS

AXIALLY LOADED SHORT SPIRAL COLUMNS

ECCENTRICALLY LOADED COLUMNS

All columns are subject to axial force and bending and they must be proportioned to resist these forces. Eccentricities of 0.10h for tied column and 0.05h for spiral columns can be permitted for axially loaded columns. Beyond this, analysis for the effect of axial load and bending must be undertaken.
Columns will tend to bend under the action of moment and produces compression on one side and tension on the other side. The following failures are possible under combined axial load and bending.

The plastic centroid of a column cross section is the point through which the resultant column load must pass to produce uniform strain in failure. It represents the location of the resultant force produced by the steel and concrete. The eccentricity of the column load is the distance from the load to the plastic centroid of the column.
In locating the plastic centroid, all concrete is assumed to be stressed in compression to 0.85f'c and all steel to fy in compression and then the centroid is then located by taking moments due to these forces.

analysis of eccentrically loaded columns

ECCENTRICALLY LOADED COLUMN WITH TWO ROWS OF REINFORCEMENT

BALANCED LOADING CONDITION

Columns normally fall by either tension or compression. In between two lies the so-called balanced load condition where failure may be either type.
Balanced loading occurs when the tension steel just reaches its yield strain as concrete is strained to 0.003. Every column has always have a loading situation where an ultimate load Pnb placed at eccentricity eb will produce a moment Mnb. If the eccentricity of the column is less than eb (e < eb) , the column fails in compression( compression controlled column); if e > eb the column fails in tension( tension controlled column).

SUPPLEMENTARY MATERIALS

20221205 - CENG132 Module 6a.pdf

20221205 - CENG132 Module 6b.pdf

recorded meetings

yga-qxqu-xkj (2022-12-05 19:07 GMT+8)

tqn-cgua-fys (2022-11-28 18:05 GMT+8)

FINAL EXAM

CamScanner 12-22-2022 20.57.pdf

REFLECTION

The last module tackled the analysis and design of reinforced concrete columns. After reading the uploaded modules, I have grasped and understood the concepts well and was able to answer the sample problems on the internet correctly. At first it was difficult for me to understand the problems without reading the concepts and step-by-step process of solving the axial load capacity of columns, especially the composite columns. However, I realized that such types of problem is quite similar to beam subjected to axial load. The formulas for me were somewhat patterned from the previous module, but has different situations depending on the cross-sectional shape of the column. Analysis of eccentrically loaded columns were too complex at first but as read them thoroughly, I was able to solve for axial load of columns with two or more rows of reinforcements. With this, I am very hopeful that the concepts that I have learned in this subject course will be very useful in my future profession as structural engineer.

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