Today, the harmony between structural elements plays an important role against earthquakes that may occur. 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 the ” 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 by 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 more healthily. 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 the “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 to create 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 the 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. To achieve this, appropriate dimensioning should be done, an adequate reinforcement ratio should be provided and attention should be paid to the ratio of transverse reinforcement (stirrups) in the joint areas.
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 by 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.
Design/implementation principles for earthquake-resistant structural systems
In planning a building, whether it is used for residence, business activities, or special functions, it must be designed according to applicable standards. This planning is of course intended to minimize the risk of failure of the structure given the condition of Indonesia which is in the Pacific Ring of Fire.
For this reason, through this article, we will discuss the principles for planning and implementing structural systems that are resistant to natural earthquakes.
Utilizing the capacity of the structure for occupant safety
Buildings and other buildings at this time must be planned not only by considering their function but must have the capacity to respond to earthquake forces that occur in each area according to the potential for an earthquake to occur.
The structure is designed must also prioritize safety and specifically allow time for residents to save themselves. In addition, a structure that conforms to the standards can minimize.
Structural aspects to implement ” Strong Column And Weak Beam “
One of the main aspects of building structure design is implementing the ” Strong Column And Weak Beam ” system, or what can be called SCWB. With the application of this system, the collapse scenario in the building can be designed in a gradual scheme (gradually) through the beam-plate and then the column parts that can experience failure.
The advantage of implementing the SCWB system is that it provides an initial signal when a building structure fails. This signal will later serve as a warning to building occupants to immediately rescue themselves. Therefore, the SCWB system is designed gradually to provide more time for residents to save themselves.
If a structure is not designed according to this scheme, of course, the bad effect is a sudden failure without any warning resulting in a collapse that can cost lives.
In general, there are several problems with building structure planning that is often encountered in the field. For example, column elements that are designed to be minimalist or too slim, lack of strong connection elements, or other errors.
Too minimalist design
The number of building structures that do not comply with the SCWB ( Strong Column And Weak Beam ) system is initially caused by an incorrect initial process. This initial process begins when construction workers design structural elements such as beams, plates, and especially in column elements where the design dimensions are too minimalist or slim.
Another common error is that the beam-column stiffness is not considered. For example, beams are strung tightly together, but columns are strung at intermittent spacing so that the lateral stiffness of the structure is not balanced or proportional. By implementing the SCWB system, the structural failure of the building can be minimized.
For more details, here we include an example of the SCWB working system applied to the form of portal deformation.
Portal Deformation Form (a) “Strong Beam” relative condition and (b) “Strong Column” relative condition. (Wongpakdee, et al, 2017)
Shallow connection details
No less interesting is the detail part of the connection. The connection elements in each building structure must be designed and designed solidly so that there is no damage to the joint area. If there is damage/weakness in the joint area, then the intact beams and plates (can also be truss frames) can suddenly break and hit the floor below. This, of course, has the potential to cause enormous casualties.
Column damage at the edge of the joint (Ning, et al, 2017)
Effect of failure
The impact of the failure of the building structure without the support of the SCWB ( Strong Column And Weak Beam ) system will be very fatal. The main point is that there is no visible stage of collapse. This means that collapse can occur suddenly, without warning, without a signal, and without giving residents more time to evacuate themselves. This is of course a very risky design, especially for occupant safety.
The following are some examples of structural failure in several countries without the application of the SCWB system in buildings.
The method of applying the SCWB system to building structures
There are several alternatives for implementing the SCWB ( Strong Column And Weak Beam ) system in building structures. The methods are as follows.
Column design dimensions must be larger than the beam
First, in the planning of building structures, the column dimensions must be made larger in comparison to the beam dimensions. This method is the basis for planning the dimension estimation in preliminary analysis.
Second, the column design should have a shape that has balanced inertia of the stiffness of the X and Y directions, it can use a square or a circle section for reinforced concrete buildings or an H section for a structural steel building.
Design the number of columns according to the area of the beam and plate
Next is to ensure that the grid or the number of columns against the span/area is a proportional condition. This is quite influential on the distribution of floor/beam loads on each column. Even distribution will provide uniform behavior so that the column can work according to the system.
Make sure beam and column joints are strong
If the column has been planned and meets the SCWB ( Strong Column And Weak Beam ) rules, the next step is to pay attention to the joint connection.
Ensure that the joint design is stronger than the column beam because the SCWB system cannot read failure scenarios for the connection. In addition, reinforced concrete must be designed according to the standard length of distribution in SNI (Indonesian National Standard). Meanwhile, steel structures can be designed by bolting, welding, or a combination of the two according to SNI.