Things to Look For In a Design of Reinforced Concrete Columns

This article discusses what things to note when designing a particular structural elements of the building structure. For this first part, the elements discussed are COLUMN.

A. Analysis

   1. Type column clamping level. If using a pedestal clamp, have confirmed its foundation strong enough to resist bending moments and keep the rotation does not occur at the lower end of column.
   2. Moment of Inertia Reduction
      For the influence of cracking columns, moment of inertia of column cross section is reduced to 0.7Ig (Ig = moment of inertia of net section)
B. Load Design (Design Loads)
Must be considered in the load used for design of concrete columns is:

   1. The loading combinations.
      As a force in Test of Concrete, Steel, and Wood.
   2. Living Expenses Cumulative reduction.
      Especially for the column (and also the axial load bearing wall), live load may be reduced by using the cumulative live load reduction factor. Reference is loading Indonesia Regulation (PBI) for Building 1983
      The table is as follows:
      Total floor carried Coefficient reduction
      1 1.0
      2 1.0
      3 0.9
      4 0.8
      5 0.7
      6 0.6
      7 0.5
      8 or more 0.4

      Example How to use:
      For example there is a column that hold the fifth floor. Each floor provides live load reaction on the column amounted to 60 kN. Then the live load used for design of columns on each floor are:
      - Level 5: 1.0 x 60 = 60 kN
      - Level 4: 1.0 x (2 × 60) = 120 kN
      - Level 3: 0.9 x (3 × 60) = 162 kN
      - Level 2: 0.8 x (4 × 60) = 192 kN
      - Level 1: 0.7 x (5 × 60) = 210 kN
      So, the bottom floor is designed for live load 210 kN, it is not necessary for 5 × 60 = 300 kN.
      The basis of this reduction pengambilkan is that small possibility of a fully burdened column by live load on each floor. In the example above, one might say that it is unlikely that column receives live load 60 kN on each floor at the same time. So that the cumulative burden may be reduced.
      Note: This expense will still have to be multiplied by load factors in a combination of loading, eg 1.2D + 1.6L.
D. In Style

   1. Forces which must be taken to design in accordance with the grouping columns, including columns rocked whether or not rocking, if included short column or columns slim.
   2. The enlargement of the moment (of order unity), and P-Delta analysis (second order) should also be considered for determining the internal forces.
C. Concrete Column detailing
For detailing, the things that need to be considered include:

   1. Sectional dimension of column.
      For columns that bear the earthquake, the smallest column size can not be less than 300 mm. Comparative dimensions of the smallest column to erect lurusnya direction can not be less than 0.4. For example a square column with the smallest size of 300mm, the size of the vertical direction should not exceed lurusnya 300/0.4 = 750 mm.
   2. Reinforcement ratio shall be not less than 0:01 (1%) and should not be more than 0:08 (8%). While for the column bearing the earthquake, the ratio is 6% maksiumumnya. Sometimes in practice, reinforcement installed less than the minimum, for example 4D13 for column size 250 × 250 (ratio 0.85%). Provided that the maximum load was far below the section's capacity, okay. But if that condition is, changing the column size to 200 × 200 with 4D13 (r = 1:33%), we think it more economical. What is important with all requirements of strength and comfort are still met.
   3. Thickness of concrete cover is 40 mm. Tolerance of 10 mm for d equal to 200 mm or smaller, and tolerance to d 12 mm greater than 200 mm. d is the reduced cross-sectional size of a thick blanket. d is the distance between the outermost concrete fiber that experienced press toward the center of the tensile reinforcement. For example the column size 300 x 300 mm, thick blankets (to the point of heavy main reinforcement) is 50 mm, then d = 300-50 = 250 mm.
      Note:
      - Tolerance of 10 mm concrete cover means may be reduced as far as 10 or 12 mm due to shifting during the installation of steel reinforcement bars. But tolerance must not be intentionally done, misanya by placing "concrete knowing" for 30-mm-thick blanket.
      - Mortar plastering and finishing the concrete cover is not included, because the mortar and finishing at any time can easily be rusted either intentional or unintentional.
   4. Pipe, channel, or a cover which is not harmful to concrete (not reactive) may be planted in the field, provided that the extent of not more than 4% of net cross-sectional area of the column, and the pipe / channel / sheath should be planted in the concrete core (inside diameter of a circle / ties / begel), not in concrete cover.
      Aluminum pipe should not be planted, unless given a protective coating. Aluminum can react with the concrete and steel reinforcement.
5. Spaces (clearance) between bars along the side of the transverse reinforcement must not exceed 150 mm.kolom_14036_image0056. Sengkang / ties / begel is an important element in the column, especially at the regional meeting of the beam-column in the resist earthquake loads. Fitting stirrup should really comply with the requirements of SNI.In addition to shear forces, transverse reinforcement is also useful for holding / megikat main reinforcement and concrete core does not "jump" when receiving a very large axial forces when an earthquake occurs, so that the column can develop resistance to the maximum limit (eg reinforced concrete began to melt or the voltage reaches 0.85 fc ')# Transfer the axial load on the structure of a quality different floors.In high-rise building, sometimes we design the columns and floor plates with different concrete quality. For example slab using fc'25 MPa and MPa fc'40 column. At the time of execution (casting floor), the column that intersect (intersection) with a floor would be casted according to the quality of the concrete slab (25 MPa). This intersection region to be checked against the axial load on it. Not uncommon in this area needed additional reinforcement to mengakomodiasi strength due to different concrete quality.