DIFFERENCES BETWEEN CLASSIC AND ISOLATED STRUCTURES
In all earthquake regulations that are currently used in the world, internationally accepted and can be called modern, the principles regarding fixed-support built-in (classical building) and earthquake isolated building design are separately. These regulations include the Eurocode (European specification), ASCE (American specifications) and which entered into force in 2019 Guidelines for Design of Under Construction Impact Earthquake (or shortly Turkey Building Earthquake Regulation – TBDY2019) are also available.
Earthquake regulations are constantly updated with continuous studies, knowledge that develops globally with the experienced earthquakes, and current technologies and approaches. For example, the Turkey Earthquake Regulations, 1998, updated in 2007 and finally in 2019. Similarly, ASCE was modernized in 2005, 2010 and 2016 according to current information and technologies.
Although other international regulations and also finds its place in the specifications in previous years, mainly related to seismic isolation Turkey Earthquake Regulations have been included for the first time in 2019. Thus, the basic requirements of the earthquake isolated building design have been determined for the conditions of our country. Although there are a few minor differences, the design principles of both earthquake isolated and built-in buildings (classical building) are very similar to each other in all these earthquake regulations.
This report, in 2019, which entered into force on Principles for the design of buildings under the Influence of Earthquake (or shortly Turkey Building Earthquake Regulation – TBDY2019) in the regulation of private summarizes information related to classic and earthquake-proof building design and performance goals.
Regulatory Design of a Classical Building Under Earthquake Effects
All regulations and specifications in under a classic structure of the design earthquake (2019 level referred to as DDR-2 in Turkey Earthquake Regulations) made damage claims acceptance and control this damage way to becoming “capacity design called” methods are applied. With the capacity design, the “strong column-weak beam” concept comes into play, and the reinforcement placements at the column-beam junction points are adjusted in accordance with this design in order to obtain structures with high ductility. In this way, it is determined in which elements of the building plastic hinges (in other words, damage) will occur under the design earthquake. As a result, structures that will be damaged under the design earthquake, but will ensure the safety of people (will not collapse) can be designed.
The force reduction coefficients (R) found in earthquake regulations are used for exactly this purpose. The forces that the building will actually feel under earthquake effects are reduced to a certain extent (according to the chosen R coefficient) and the design is made accordingly. The basis of this approach is based on the assumption that the structural elements will move to the part where they will behave as “plastic”. That is, structural elements will not behave elastic during an earthquake and will be damaged. This damage is controlled by the capacity design and it is ensured that the building does not collapse. All earthquake codes adopt this approach for conventional structures.
Turkey Earthquake 2019 were identified four design earthquake levels in the Regulation. The descriptions of earthquake levels are shown in Figure 1. The target performance levels that the buildings can provide under the effects of earthquakes are presented in Figure 2, and the performance targets that the building should meet depending on the classical building type are presented in Figure 3. All of these figures are taken from the Principles for the Design of Buildings Under Earthquake Effect document, which entered into force in 2019.
As it can be understood from the articles in Figure 1, the design earthquake level of a classical building is determined as minimum DD-2. Considering the explanations in Figure 2 and the table in Figure 3, for a “new cast-in-place reinforced concrete” structure in item (a) of the table, if the building is designed according to the “advanced” performance target, the performance target of the building is both DD-1 and For earthquakes at DD-2 levels, KH, that is Controlled Damage, in other words, is “Safety of Life”. This is the minimum target allowed by the regulation, and a further performance target can be selected at the request of the building owner. However, all standard practices are carried out with the performance target of “Controlled Damage”.
In other words, both structural and non-structural elements in a classical structure designed in full compliance with the most up-to-date earthquake regulation and meeting all application requirements in the same regulation will be damaged (KH) when the earthquake (DD-2) it is designed for. This level of damage may be light, medium or severe, but the structure will not collapse and the people inside can be evacuated safely. The building will not be usable after the earthquake. Depending on the level of damage, it may need to be repaired or destroyed completely.
Design of an Earthquake Isolated Building Under Earthquake Effects According to the Regulation
Contrary to the classical structure, R coefficients in an earthquake isolated building are taken as very low (1.2, 1.5 etc.). In this way, it is ensured that structural elements survive the design earthquake with zero damage. Similarly, the seismic isolation devices and structural members beneath the seal apparatus (e.g., base and base-top columns) under maximum earthquake (2019 level indicated as DD-1 Turkey Earthquake Code) to be completely elastic (R = 1) is contemplated. Thus, the structural elements (column, beam, foundation, curtain, slab) in the entire structure survive any earthquake with zero damage. In Figure 4, 2019 in Turkey Earthquake Regulations it is situated performance targets related to the earthquake-proof buildings.
As can be seen from the table in Figure 4, earthquake proof structures are designed at the level of DD-2, which is the design earthquake, with KK (Continuous Use), ie zero damage principle, for the “advanced” performance target. Earthquake isolation devices and infrastructure are designed to be even more superior, with the goal of Uninterrupted Usage even under the maximum expected earthquake in that region. Thus, the infrastructure elements, isolation devices and the superstructure itself survive the maximum expected earthquake in that area without damage and the building continues to use.
In addition, earthquake isolation significantly reduces the floor accelerations to the building. This can be explained by the basic working principle of earthquake isolation. Accelerations felt in the upper structure and relative floor displacements remain at very low levels thanks to both the isolation system having a large period and the high damping rates that can be achieved with isolation devices. This enables non-structural elements to survive a maximum possible earthquake with zero damage. Details of this behavior are shown in Figure 5.
In other words, an earthquake-proof building designed and applied in accordance with the regulations will survive the expected design earthquake in that region completely without damage. In addition to the fact that the building does not need any repair, the building will be able to continue to be used uninterruptedly.
An illustration about the effect of R coefficients found in earthquake specifications and regulations on building design is given in Figure 6. This figure is for approximate values and may differ for each structure, but describes the basic principle. In this way, the pink curve is the acceleration spectrum created for a region. Green curve is the design spectrum obtained by reducing the pink curve according to the ductility level of the structure using R coefficients. The yellow curve is the acceleration spectrum obtained as a result of the earthquake isolation application, without using any other reduction coefficient. Also, the natural periods of a classical building up to 10 floors do not exceed 1.5 seconds, that is, it feels the highest accelerations (and therefore forces) in a possible earthquake. Earthquake isolation automatically increases the period of the superstructure to 2 seconds or more. In addition, since it increases the damping ratio of 5% to 20% and above, it enables the buildings to be designed as completely elastic.
1- According to the 2019 Turkish Earthquake Regulation, residential buildings that are in the BKS = 3 class should meet the “controlled damage” performance target under the design earthquake (DD-2).
2- This performance level, formerly known as “life safety”, only aims the evacuation of people after a possible earthquake.
3- This situation means that the building is likely to become unusable or even non-reinforced after an earthquake.
4- The expected performance target in earthquake isolated buildings is “continuous use” in the design earthquake (DD-2) and “controlled damage” in the biggest earthquake (DD-1).
5- For this reason, isolated buildings can be used immediately after an earthquake, and structural and non-structural damage will almost not occur.
6- Again, for this reason, the contents of the building will not be damaged in isolated buildings.