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The Effect of Short Columns on Stability and Robustness.

posted Mar 17, 2022, 10:18 PM by jeffery jim
Contradict to common believe that slender column is most likely to fail, short column is usually the point of failure when it comes to lateral movement or force, not vertical movement or force. Hence, a field study was conducted post Sabah Mega Earthquake yield a very interesting finding and now a case study for many engineering schools.

1. Column robustness depends on the reinforcements that entail the behavior of the structural integrity as a whole. Short or long/slender column performs accordingly if it is designed to resist lateral force depending on the ductility class.

2. Under compression, short columns perform better than slender columns because of the sturdiness as long as the concrete is taking the load with high compressive strength.

3. Under lateral force, slender columns offer elasticity and bending in comparison to shorter columns which is also rigid with high ductility coming from the reinforcement bars. Tensile ability of the reinforcement bar plays an important role when it deforms due to displacement.

4. Under uni-axial loading coming from both self-weight and transient loads reacting on the structure during an earthquake event; the column is exposed to many element such as force, moment and even torsion force.

5. Both short column and slender column will fail but the nature or mode of failure differs. Slender column will fail, however the nature of failure is not be rapid but will increase by cycles and yield strength of the reinforcement bar give up slowly and will have sufficient time to resist by entering into the elastic phase. Unlike short column, the reaction is instantaneous where at certain case, the reached the ultimate limit state without entering the deflection stage and capitalize on yield strength to resist lateral forces.

6. The issue which is not tackled here is the ability to be shear resistant when extreme lateral forces were introduced. By introducing shear reinforcement bars and increasing the rigidity of the structure, the structure will find the nearest point or structural node to fail and relief.

7. The introduced spectrum may not be long but it all depends on the ground when it comes to earthquake engineering. Very strong ground may not cause wobbling effect to the structure as tall and slender structures on soft ground (worst, with geological deposits). In harder ground, the acceleration will be higher and time is lesser and vice versa. F=M.a. Force equals to mass multiple with acceleration and hence, the force will increase.

8. However, as engineers, to compare a short column to a slender column is futile as the duty of a structural engineer is to ensure forces introduced can be suppress through two earthquake engineering concepts; by force or by energy (dissipation or damping).

9. IEM prefer energy dissipation but I side Prof Dr. Azlan Adnan (UTM). Therefore, I will recommend the use of rigid structural allowable deflection as the concept to go with knowing that reinforcement bars with high tensile and yield strength is the way forward. The reason why energy dissipation is not my preference is simply due to the degradation of petro-chemical products or by-products when exposed to UV or other agents such as bacillus that may ingress to alter the molecular structure or bonding of these products.

10. I hope this short note will give you fundamental understanding about column's dimension and its respond when uni-axial forces are introduced.