Ductility of Building Structures for Earthquake Resistant Design

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Ductility for earthquake resistant design is important for buildings, structures and building materials. Ductility and its importance in design is discussed. To understand the importance of ductility in influencing the building performance, it is essential to know what is ductility. Ductility in general gains a definition in material engineering science as the ratio of ultimate strain to yield strain of the material. In a broader view, we must understand ductility as the ability of a structure to undergo larger deformations without collapsing. As per special provisions recommended in codes, the detailing of the structure that lets the structure to gain a larger ductility other than the contributions of material ductility are called as ductility detailing or ductile detailing. When a structure undergoes dynamic forces (which is considered as the seismic demand) the structure could not remain elastic anymore, and the next stage is damage. It can go through plastic stage or fracture or damage, where stiffness will decrease appreciably and deformations will be drastically increasing even for a small load. These situations must be expected by an engineer and we should ensure that our design sustains these loads without undergoing larger deformations or no collapse. For this target to be achieved, we should incorporate or increase the ductility of the building.

Ductility of Structural Materials

Ductility of Brick Masonry

One of the commonly used materials for building construction is masonry that is made from burnt clay bricks as well as cement mortar. In areas with the plain terrain, mud mortar is used instead of cement mortar due to difficulty in resources. Now considering their structural properties, they can undergo higher compression but they are weak in tension as shown in the figure below.

Fig.1. Masonry Strong in Compression and Weak in Tension

Here compression implies pressing together the ends of the wall, that is easily resisted by the masonry work. But when tension is applied that is, a pulling force to the masonry elements would not make the masonry in good condition.

Ductility of Concrete

Concrete is another material that is used widespread in construction over the past 4 decades. The composition, as we are known about is cement, aggregates, and water. Here aggregates are finer and coarser ones and all mixed together with the appropriate proportion of water. But again, concrete is good in compression and weak in tension. Concrete gains a high compressive strength compared to masonry, but are weak for a pulling force or so-called tension. Even though the strength of concrete depends on the water proportions, either high or low would affect the concrete mix and hence strength. But concrete and brick masonry are considered more brittle and fails suddenly.

Ductility of Steel Reinforcement

Steel which is one of the strongest material used in construction is used in concrete and masonry buildings in the form of reinforcement bars. They are available in the market in different diameters. Mostly 6mm to 40mm diameter bars are used for reinforcing. Steel reinforcement implementation is due to the property steel gain, i.e. it is good in both compression and tension. And above all steel is a ductile material. This property has helped construction use steel for increasing the ductility of buildings. This in structural elements helps to undergo longer elongation without undergoing collapse.

Ductility of Reinforced Concrete

Then we have concrete, which is used in buildings along with steel reinforcement which gives reinforced cement concrete. Hence R.C.C forms a composite material. The placement of steel reinforcement is done such a way that, under the action of ultimate load steel must reach its maximum strength in tension before concrete reaches its maximum strength in compression. This would result in ductile failure of the structure. In certain structure, ductile failure is expected intentionally. But this does not mean that additional steel reinforcement would play well. Too much steel reinforcement is considered harmful.

Need for Ductile Reinforcement in Building Structures

If the structure can stay elastic under the action of the maximum expected earthquake in the respective area, then there is a need for ductile reinforcement. But increasing the elastic property with the demand was found as an uneconomical method of construction method even not appropriate for emerging with architecturally viable design ideas. For having economy and safety for building structures under earthquake motions which are unexpected, the method opted is to let structure undergo damage either by means of plasticity, fracture, crushing etc. keeping its strength to carry the vertical load as it is undergoing deformations during damage. For example, we will consider columns that must be designed for ductility. The method is to provide confine concrete with reinforcement to avoid buckling of longitudinal reinforcement. This would enable the column to continue taking vertical loads even if it is subjected to cracking, concrete crushing or yielding of steel reinforcement. Now this arrangement would cause the material to become compliant, hence the stiffness force values in the structures and related components would drop. That is if the structure had remained elastic there would be higher internal forces and total base shear. Here the total base shear can be explained as the sum of internal shear forces in all the vertical load-carrying structural elements. So, incorporation of ductility in the material means, we have allowed the structure to get damaged, thus less internal forces. This provision is based on Dynamic Response Modification Factor or ductility factor, which is provided in building codes worldwide. The ductility factor depends on upon the lateral structural system implemented. For systems that have to undergo larger deformations without collapse when damage occurs would have a higher ductility factor. Thus, for a structural system that has to undergo only small deformations before collapsing will gain only small ductility factor.

Capacity Design Concept for Increased Ductility of Structures

The above explained criteria is defined as a capacity design method, the phenomenon of ductility incorporation in building elements. This considers the problem of determining the failure mechanism of members. The idea is to force the member to undergo failure in a ductile manner by making the capacity of the member in other possible failure modes greater. Consider two bars of the same length and cross-sectional area. Here one of them forms a ductile material and other one forming brittle. Pulling these two bars on either side as shown in figure 2, will make them break under extreme load. It is observed that the ductile material elongates for a larger amount and the brittle material breaks for a small elongation. Hence among the materials used steel is more ductile than concrete which is brittle.

Fig.2: Ductile Material vs. Brittle Material

Ductile Chain Design concept in building as per capacity design. When a brittle chain alone is pulled on either side, it would break suddenly. Among the chain, the weakest link would break first. If we make the weakest link as the ductile one, we can gain more elongation and more ductile. In buildings, we implement the same concept in the form of a ductile chain. The seismic inertia forces are transmitted from the floors to the beams then to the columns. The failure of the column would affect the stability of the building than the beam failure. Hence it is appreciable to make beams as weak ductile links than columns. This design method is called as strong column-weak beam concept.

Fig.3. Ductile Chain in Capacity Design Concept