BST – 10346 Structural Design – Short Column Design 1

Short Column Design In most reinforced concrete buildings, the imposed and dead loads are usually supported by slabs which span between beams; these beams in turn span between columns. Columns carry the loads down to the ground level and eventually to the foundation and firm stratum. Reinforced Concrete Columns The reinforced concrete columns in general use are round, square or rectangular. For an axially loaded column without bending, unless its shape is dictated by architectural features, the section would be round or square. Round columns are slightly more expensive because of the special formwork required. For a column with bending, an economical section would be rectangular, with its longer side parallel to the plane of bending. Braced and Unbraced Columns Column may be braced or unbraced A braced column in a given direction is one where the lateral loads in that direction are totally taken by shear walls or other suitable bracing in the same direction. An unbraced column in a given direction is one where the lateral loads in that direction are totally taken by the columns themselves. According to BS 8110, a braced column is defined as follows: “A column may be considered braced in a given plan if lateral stability to the structure as a whole is provided by walls or bracing designed to resist all lateral forces in that plane. It should otherwise be considered as unbraced”.

BST – 10346 Structural Design – Short Column Design 2

For the y-direction of the above diagram, the lateral stability of the building is provided by the shear walls. The columns can thus be considered to be braced in the y-direction. In the x-direction, the lateral stability of the building is provided by the columns, together with shear walls bending about their weaker axis. The columns are therefore said to be unbraced in the x-direction. Short and Slender Column A reinforced concrete column will be classified as a short column when its effective height is less than 15 times its width or breadth in a braced column and less than 10 times its width or breath in an unbraced column. In most of the cases, the column will be designed as “Short Column” in order to prevent “Buckling” failure.

Worked Example Design the minimum dimension of short column with effective height 2.8 m. Solution:

2800 / 15 = 186 mm, say 200 mm

Therefore, the minimum size to be 200 mm.

BST – 10346 Structural Design – Short Column Design 3

Column Design Column sections should normally be designed using the design chart (discuss later). Alternatively, the following simplified procedure may be adopted where applicable:

a) In the case of columns where only the minimum design moment applies, the ultimate axial load capacity in N of the moment may be taken as:

N = 0.4fcuAc + 0.75fyAsc

Where N = ultimate axial load Ac = area of concrete section Asc = area of steel in the concrete section b) In the case of columns supporting an approximately symmetrical arrangement of beams,

subject to uniformly distributed loads, the ultimate axial load capacity of the column may be taken as

N = 0.35fcuAc + 0.67fyAsc

Column Links When the longitudinal steel bars in a column are subjected to compression, they will tend to buckle, and in so doing will cause the concrete to spall off. The column will thereby lose part of its sectional area, and the steel will also be exposed to weather and will corrode. To prevent this possible form of failure, links must be provided to tie in all the main steel bars, and they must be placed sufficiently close together to prevent the bars from buckling between the links.

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Column Design Chart It is the most accurate method to design the column whatever square, rectangular or circular by mean of manual. Here are the procedures: 1) Determine the N (Ultimate axially force) and M (Ultimate moment) from the working

drawings. 2) Choosing the suitable design chart to find the ρfy/fcu value, where ρ = Asc/bh 3) The maximum percentage of longitudinal reinforcement in terms of the gross

cross-sectional area of the concrete should not exceed: ‧ 6% for vertically cast columns ‧ 8% for horizontally cast columns ‧ 10% at laps in columns

BST – 10346 Structural Design – Short Column Design 5

Worked Example Design the short column with the following data: fcu = 30 N/mm² fy = 460 N/mm² b = 300 mm h = 450 mm N = 3000 kN M = 450 kNm Nominal cover = 20 mm Solution: Assume main bar diameter to be 32 mm. d = 450 – 20 – 32/2 = 402 mm d/h = 402 / 450 = 0.9 N/bh = 3000 / (300 x 450) = 22 M/bh² = 400 / (300 x 450²) = 6.58 From design chart, 100Asc /bh = 6 Asc = 6 x 300 x 450 / 100 = 8100 mm² Provide = 4T40 at top and bottom = 5040 mm² > 8100/2 = 4050mm² OK

From alternative design chart, N/bhfcu = 3000 / (300 x 450 x 30) = 0.74 M/bh²fcu = 400 / (300 x 450² x 30) = 0.22 ρfy/fcu = = 0.92 ρ = 0.92 x 30 / 460 = 0.06 Asc = 0.06 x 300 x 450 = 8100 mm² Provide = 4T40 at top and bottom = 5040 mm² > 8100/2 = 4050mm² OK

BST – 10346 Structural Design – Short Column Design 6

BST – 10346 Structural Design – Short Column Design 7

Worked Example Design the short column with the following data: fcu = 30 N/mm² fy = 460 N/mm² Ø = 1500 mm N = 40000 kN M = 11000 kNm Nominal cover = 100 mm Solution: Assume main bar diameter to be 40 mm. hs = 1500 – (100 x 2-10 x 2-40) = 1240 mm hs/h = 1240 / 1500 = 0.8 N/h²fcu = 40000 / 1500² / 30 = 0.6 M/h³fcu = 11000 / 1500³ / 30 = 0.1 From design chart, w = 0.88 Asc = 0.88 x 30(fcu) x 750²π(Ac) / 460(fy)

= 101419 mm² Provide = 52T50 = 102076 mm² > 101419 mm²

OK

From alternative design chart, ρfy/fcu = = 0.88 Asc = 0.88 x 30(fcu) x 750²π(Ac) / 460(fy)

= 101419 mm² Provide = 52T50 = 102076 mm² > 101419 mm²

OK

On completion of this topic, you should be able to: 1. Define braced and unbraced columns. 2. Define and calculate the effective height and end conditions of a reinforced concrete

column 3. Define the short and slender columns. 4. Short column design