Structure II Course Code: ARCH 209 Dr. Aeid A. Abdulrazeg

Short column design The short column are divided into three categories: Columns resisting axial load only, Columns supporting an approximately symmetrical arrangement of beams, Columns resisting axial loads and uniaxial or biaxial bending

It supports a symmetrical arrangement of beams but which are unequal in length. If (a) the loadings on the beam are uniformly distributed, (b) the beam spans do not differ by more than 15 percent, the column C2 belongs to category 2. It will resist an axial load only, as it supports beams equal in length and symmetrically arranged. If the column does not meet criteria (a) and (b), then the column belongs to category 3.

Both longitudinal steel and concrete assist in carrying the load: For pure axial load: Where: Ac is the net area of concrete. Design maximum axial load capacity of short column is:

To allow for nominal eccentricity, BS 8110 reduce the theoretical axial load capacity by about 10%.

Example 1: Short column design Determine the load capacity of the shown column. 300 mm 4T25 300 mm

Example 2: Short column design The load capacity of the column is 1366 kN and column size as shown in figure. Find the area of steel reinforcement is required. 250 mm 250 mm

Column Resisting an Axial Load and Uniaxial Bending

Construction of Design Chart A design curve can be drawn for a selected grade of concrete and reinforcing steel for a section with a given percentage of reinforcement, 100Asc/bh, symmetrically placed at a given location d/h. The curve is formed by plotting values of N/bh against M/bh2 for various positions of the neutral axis x. Other curves can be constructed for percentages of steel ranging from 0.4% to a maximum of 6% for vertically cast columns. The family of curves forms the design chart for that combination of materials and steel location. Separate charts are required for the same materials for different values of d/h which determines the location of the reinforcement in the section. Groups of charts are required for the various combinations of concrete and steel grades. Design charts are given in Part 3 of the code. The process for construction of a design chart is demonstrated below.

Design chart for column resisting an axial load and uniaxial bending moment, (Part 3, BS 8110)

The area of longitudinal reinforcement should lie in the limits: Reinforcement Details: Longitudinal Reinforcement (clause 3.12.5, BS 8110) Size and minimum number of bars – bar size should not be less than 12 mm in diameter. Rectangular column should reinforced with minimum 4 bars; circular column should reinforced with minimum 6 bars. The area of longitudinal reinforcement should lie in the limits: Vertically cast Horizontally cast

Reinforcement details: longitudinal reinforcement (clause 3. 12 Reinforcement details: longitudinal reinforcement (clause 3.12.5, BS 8110) Spacing of reinforcement – the minimum distance between adjacent bars should not be less than the diameter of the bar or hagg + 5 mm.

Links are passing round the bars to prevent buckling. Reinforcement details – links (clause 3.12.7, BS 8110) The axial loading on the column may cause buckling of the longitudinal reinforcement and subsequent cracking and spalling of concrete cover. Links are passing round the bars to prevent buckling.

Reinforcement details – links (clause 3.12.7, BS 8110) Size and spacing of links – the diameter of the link should be at least one quarter of the largest longitudinal bar size or minimum 8 mm. The maximum spacing is 12 times of the smallest longitudinal bar. Arrangement of links

Example 3: Short column design A short braced column is subjected to an ultimate load of 1480 kN and ultimate moment of 54 kN.m. the column section is 300 mm x 300 mm. Determine the area of steel required. Assume 25 mm diameter bars for main reinforcement and 8 mm diameter links. The cover on the links is 25 mm.

Example 3: Short column design BS8110- Chart No. 28

Provide 4T25mm to give an area of 1963 mm2 Links: The diameter of the links is one – quarter times the diameter of the largest longitudinal bar, that is The spacing of the links is the lesser of (a) 12 times the diameter of the smallest longitudinal bar, that is Or (b) the smallest cross- sectional dimension of the column That is 300 mm Provide T8 @ 300 mm