Failure Concrete Column

၁၆၈ ၊ Failure of Concrete Column

Concrete column မ်ား၊ ပ်က္စီးပံု၊ ၃-မ်ိဳး၊ ရွိပါသည္။
a. Pure Compression failure
b. combined compression and bending failure
c. Elastic Instability failure

1. Column ဆိုတာ၊ compression member ျဖစ္ပါသည္။ ၄င္းသည္၊ ၄င္းသည္ အေဆာက္အဦး၏၊ အေလးခ်ိန္ ကို၊ ထမ္းေဆာင္ၿပီး၊ foundation မ်ားကို၊ ျဖတ္သန္းေစကာ၊ ေျမႀကီးသို႔၊ ပို႔ေဆာင္ေပးပါသည္။ column ၏၊ အျမင့္ႏွင့္ ျဖတ္ပိုင္းပံုအရ၊ column အမ်ိဳးအစား၃မ်ိဳးခြဲျခားႏိုင္ပါသည္။
ေကာ္လံအျမင့္ကို၊ ျဖတ္ပိုင္းပံု ငယ္ေသာအနားႏွင့္စားေသာ္၊ ရလာဘ္သည္_
၁။ ၁၂ ထက္ႀကီးပါက၊ long column ဟုလည္းေကာင္း။
၂။ ၃ ထက္ငယ္ပါက၊ pedestal  column ဟုလည္းေကာင္း။
၃။ ထိုၾကားတြင္ရွိပါက၊  short column ဟုလည္းေကာင္း။ ေခၚပါသည္။

2. Column တို႔သည္၊ slenderness ratio ေပၚမူတည္ၿပီး၊ အေလးဝန္ခ်ိန္ကို၊ ထမ္းႏိုင္သလို၊ ပ်က္စီးပံုလည္း ကြဲျပားေလ့ရွိၾကပါသည္။ slenderness ratio ဆိုသည္မွာ၊
2.1 Indian and British Standard က
effective length - Le ကို၊ ျဖတ္ပိုင္းပံု ငယ္ေသာအနားႏွင့္ စားလို႔ရေသာ"ကိန္း"ဟု၊ သတ္မွတ္ျပဌာန္းပါသည္။
2.2 American Concrete Institute Code of Practice က
effective length - Le ကို၊ radius of gyration ႏွင့္ စားလို႔ရေသာ"ကိန္း"ဟု၊ သတ္မွတ္ျပဌာန္းပါသည္။
2.3 column တလံုး၏၊ Effective length သည္၊ ထို ေကာ္လန္၏၊ အစြန္းႏွစ္ဘက္ရွိ၊ support မ်ားေပၚ မူတည္ၿပီး၊ ကြဲျပားျခားနားတပ္ပါသည္။

3. Failure of Concrete Column
"Column တို႔သည္၊ slenderness ratio ေပၚမူတည္ၿပီး၊ အေလးဝန္ခ်ိန္ကို၊ ထမ္းႏိုင္သလို၊ ပ်က္စီးပံုလည္း ကြဲျပားေလ့ရွိၾကပါသည္ဟု" အထက္တြင္၊ တင္ျပခဲ့ၿပီးျဖစ္ပါသည္။ slenderness ratio ေပၚမူတည္ၿပီး၊ ေကာ္လန္ ၃မ်ိဳး ခြဲျခားထားသလို၊ ပ်က္စီးပံုလည္း ၃မ်ိဳးရွိပါသည္။ load ကို၊ ဗဟိုက် load ဟု၊ ယူဆပါ။

3.1 Pure Compression failure - ကြဲေက် ။
column သည္၊ axial load ေၾကာင့္၊ steel ေရာ concrete ပါ၊ yield stress ကို၊ ခံစားရ ေသာအခါ၊ lateral deformation - ေဘးတိုက္ပံုေျပာင္းျခင္းမျဖစ္ဘဲ၊ ပ်က္စီးျခင္းကို၊ Pure Compression failure ဟု၊ ေခၚပါသည္။ ၄င္းသည္ material failure ေၾကာင့္၊ ကြဲေက် ပ်က္စီးသြားျခင္းျဖစ္ပါသည္။ ( ထိုသို႔ေသာ ပ်က္စီး ျခင္းမ်ိဳးကို၊ ေရွာင္လြဲရန္အတြက္၊ ေကာ္လန္၏ cross-sectional area ကို၊ လံုေလာက္ေအာင္၊ ႀကီးေပးရပါမည္။) pedestal column မ်ားတြင္၊ ဤပ်က္စီးမႈမ်ိဳး၊ ျဖစ္ေပၚတပ္ပါသည္။ axial load ေၾကာင့္ bending ကို၊ ခံစားရၿပီး၊ ပ်က္စီးရျခင္း၊ မဟုတ္ပါ။

3.2 combined compression and bending failure - ေကြး
Short column မ်ားသည္၊ axial loads, lateral loads ႏွင့္ moment မ်ားကို၊ ခံစားၾကရပါသည္။ ၄င္း Short column မ်ားသည္၊ lateral load ႏွင့္ moment ေၾကာင့္၊ lateral deflection ႏွင့္ bending ျဖစ္ေပၚလာပါ သည္။ Long column မ်ားသည္၊ axial load တခုတည္းေၾကာင့္ပင္လ် င္၊ lateral deflection ႏွင့္ bending ျဖစ္ေပၚလာႏိုင္ပါသည္။ ထိုသို႔ေသာ၊ အေခ်အေနတြင္၊ steel ေရာ concrete ပါ၊ yield stress ကို ခံစားရေသာေၾကာင့္၊ column တိုင္၊ ပ်က္စီးရျခင္းကို၊ combined compression and bending failure ဟု၊ ေခၚပါသည္။

3.3 Elastic Instability failure - ဗိုက္ပူ
Long column တို႔သည္၊ ပိန္ပိန္ရွည္ရွည္ျဖစ္ၾကေသာေၾကာင့္၊ concrete x-section area ႏွင့္ steel area အတိုင္း၊ အေလးဝန္ခ်ိန္ ထမ္းေဆာင္ႏိုင္ျခင္း၊ မရွိၾကေပ။ အဆိုပါ၊ long column တို႔သည္၊ load အနည္းငယ္ သက္ေရာက္ရံုမၽွျဖင့္၊ မတည္မၿငိမ္ျဖစ္လာၿပီး၊ မည္သည့္ဘက္ကိုမဆို၊ buckle - ဗိုက္ပူသြားတပ္ေပသည္။ ထိုသို႔ load အနည္းငယ္တင္ရံုမၽွျဖင့္၊  lateral elastic buckling - ေဘးသို႔၊ ဗိုက္ေဖာင္းလိုက္၊ ပိန္လိုက္၊ ျဖစ္ေပၚလာကာ၊ reinforcement steel ေရာ concrete ပါ yield stress ေရာက္၍၊ ပ်က္စီးရျခင္းကို၊ Elastic Instability failure ဟု၊ ေခၚပါသည္။ ထိုသို႔ေသာ ပ်က္စီးျခင္းမ်ိဳး မျဖစ္ေပၚေအာင္၊ slenderness ratio ၃၀ ထက္ေက်ာ္ေသာ၊ေကာ္လန္မ်ားကို၊ တည္ေဆာက္ျခင္းမွ၊ တားျမစ္ထားပါသည္။

1. Reinforced concrete column is a compression member and transfers the loads from structure to the ground through foundations. There are three types of concrete columns based on its height and lateral dimension. Long columns are those whose ratio of height to least lateral dimension is more than 12. When the height to least lateral dimension is less than 3, it is called a pedestal and if it is between 3 and 12, it is called as a short column.

2. The load carrying capacity and modes of failure of a reinforced concrete column is based on the slenderness ratio. Slenderness ratio is the ratio of the effective length Le and least lateral dimension of the column as per Indian and British Standards. But as per American Concrete Institute Code of Practice, the slenderness ratio is defined as the ratio of effective length of column to its radius of gyration, which is same as used for structural steel design as per IS Code. Effective length of a column depends on its support conditions at ends.

3. Failure of Concrete Column
Based on the slenderness ratio of the column, there are three modes of failure of reinforced concrete columns. The columns are assumed to be centrally loaded (no eccentric loads).

Mode – 1: Column Failure due to Pure Compression:
When reinforced concrete columns are axially loaded, the reinforcement steel and concrete experiences stresses. When the loads are high compared to cross-sectional area of the column, the steel and concrete reach the yield stress and column fails without undergoing any lateral deformation. The concrete column is crushed and collapse of the column is due to the material failure. To overcome this, the concrete column should have sufficient cross-sectional area, so that the stress is under the specified limit. This type of failure is generally seen in case of pedestals whose height to least lateral dimension is less than 3 and does not experience bending due to axial loads.

Mode – 2: Column Failure due to Combined Compression and bending:
Short columns are commonly subjected to axial loads, lateral loads and moments. Short columns under the action of lateral loads and moments undergo lateral deflection and bending. Long columns undergo lateral deflection and bending even when they are only axially loaded. Under such circumstances when the stresses in steel and concrete reach their yield stress, material failure happens and RCC column fails. This type of failure is called combined compression and bending failure.

Mode – 3: Column Failure due to Elastic Instability:
Long columns are very slender, i.e. its effective length to least lateral dimension is more than 12. Under such condition, the load carrying capacity of reinforced concrete columns reduces drastically for given cross-sectional area and percentage of reinforcement steel. When such type of concrete columns are subjected to even small loads, they tend to become unstable and buckle to any side. So, the reinforcement steel and concrete in such cases reach their yield stress even for small loads and fail due to lateral elastic buckling. This type of failure is unacceptable in practical concrete constructions. Code prevents usage of such long columns for slenderness ratio greater than 30 (for unbraced columns) for the use in concrete structures.

Fig: Failure Modes of Concrete Columns For Different Slenderness Ratio
credit Sir kyawlwintun