Today, the harmony between structural elements plays an important role against earthquakes that may occur. In order to ensure this harmony, there must be a relationship between the dimensions of the carrier elements. In this article, we will examine this relationship between carrier elements by detailing the subject of ” Strong Column Weak Beam Principle “.
The vulnerability of buildings and the damage caused by the earthquake are proportional. The dimensions of the damages occurring in buildings that are not adequately secured and are not made in accordance with the conditions specified in the regulations are more.
Rigidity, strength and ductility are the main factors to consider in reinforced concrete building design . One of the conditions required to provide these principles is sizing. The dimensional compatibility of the carrier elements with each other has an important place in the earthquake resistant building design specifications.
Bearing structure elements with dimensional harmony between them perform the load transfer in a healthier way. Since column-beam joints are obtained without using joining tools in reinforced concrete structures, there should be a relationship between the dimensions of these carrier elements.
Earthquake regulations stipulate that columns are designed to be stronger than beams. This is called “Strong Column Weak Beam Principle” .
Strong Column Weak Beam Principle Concept
The dimensional harmony between column , beam , slab and foundation , which is the carrier system of a building , positively affects the behavior of the structure under horizontal and vertical loads.
It is required that the capacity of the columns from the elements constituting the carrier system is at least 20% more than the capacity of the beams. However, nowadays, columns are often declared as architecturally undesirable elements, either reduced in size or removed entirely. Columns are made weaker than beams, especially for the purposes of creating large areas.
Beams are required to precede columns among the elements that will undergo deformation under horizontal loads. Because the collapse of the beam causes partial damage to the structure, while collapse of the column affects the entire structure. One of the important concepts in this regard is plastic articulation.
What is Plastic Hinge?
“Points where structural elements carry momentum up to a certain capacity and then allow rotation are called points where plastic hinges are formed.” The difference between the plastic joint and the normal joint is that it has a constant moment on it. In order to achieve this, appropriate dimensioning should be done, adequate reinforcement ratio should be provided and attention should be paid to the ratio of transverse reinforcement (stirrups) in the joint areas.
Conditions for Columns to be Stronger than Beams
In building carrier systems, the sum of the bearing moments of the carrier columns at the node point is required to be at least 20% more than the total bearing moments of the carrier beams at that point.
R a : An ultimate moment, calculated on the lower end of the free length of the column
Mri : the ultimate moment, calculated at the upper end free length of the column
Mri : beam left end i in the column or wall face in the calculated ultimate moment of
Mrj : beam right end j of the column or wall face It
is defined as the calculated bearing strength moment.
NOTE: Here, for the column moment carrying capacity calculation, the
the normal force of the combination that makes the column moment the lowest should be selected from the action-capacity conditions .
Solutions Against Weak Column Condition
As a result of the construction of the beams together with the flooring, the frame damage occurs more in the columns as a result of the high bearing strength.
Column carrying capacity is increased by retrofitting for weak columns. In these studies, it is generally strengthened with various methods such as column, jacketing. However, in these methods, the limited usage area in the project is reduced by enlarging the column section.
Building systems that do not contain irregularities, are designed in accordance with regulations and have been inspected effectively during the construction phase can behave ductile even in a very severe earthquake and the damage they can take can be limited.