Expansion Joints
An expansion joint is a joint provided in a building to mitigate the risk of crack formation due to thermal expansion. The joint allows separation of the building that will allow the building to expand and release stresses thereby preventing the release of stresses by cracking. These joints are provided to accommodate the expansion of adjacent building parts and relieve compressive stresses that may otherwise develop.
Expansion joints essentially provide a space between the parts and may sometimes be provided with the load transmitting devices between the parts and generally filled with expansion joint filler which is compressible enough to accommodate the expansion of adjacent parts and having the ability to regain 75 percent of the original thickness, when pressure is released.
Seismic Joints
Seismic joints are frequently required between adjacent buildings and are often introduced to separate two or more parts of the same building. Seismic joints occur naturally when one building is built adjacent to another, whether or not the buildings are linked functionally. Seismic joints are also frequently introduced to separate wings, or other parts of a single building. A seismic joint typically creates a separation between the adjacent buildings or parts of buildings that includes the separation of walls, floors, and roofs.
Differences between Expansion Joints and Seismic Joints
Seismic joints are similar to expansion joints, but at the same time very different. Expansion joints are introduced to accommodate building movements caused by shrinkage, creep, or temperature changes. They are often one-way joints, that is, they are primarily intended to accommodate movements in the direction perpendicular to the joint. Expansion joints are also commonly placed at some regular interval of length based on the expected rate of shrinkage or temperature movement expected to occur over the building length. Seismic joints, on the other hand, must accommodate movement in both orthogonal directions simultaneously and their spacing is not typically affected by building length or size.
To understand it in another way expansion joints can be imagined as the joint that is to be provided due to longitudinal expansion in a plan 2D plane, whereas, seismic joints can be imagined as the joint that is to be provided due to the lateral deformation in elevation 2D plane.
Source: Guidelines for seismic vulnerability of hospitals https://www.preventionweb.net/files/1954_VL206311.pdf
Building Configuration and Need of the Joint
“If we have a poor configuration to start with, all the engineer can do is to provide a Band-Aid – improve a basically poor solution as best as he can. Conversely, if we start off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much.”
- Er. Henry Degenklob, USA
Horizontal layout of buildings
Buildings with simple geometry in plan perform well during strong earthquakes. Building with re-entrant corners, like U, V, H, and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings into parts by using a separation joint at the junction.
Source: https://theconstructor.org/structural-engg/architectural-features-seismic-resistance/2719/
Re-entrant corners
The re-entrant, lack of continuity, or inside corner is the common characteristic of overall building configurations of an L, T, H, and + due to lack of tensile capacity and force concentration. According to the code, plan configurations of a structure and its lateral force-resisting system contains re-entrant corners, where both projections of the structure beyond the re-entrant corner are greater than 15 % of its plan dimension in the given direction. The re-entrant corners of the buildings are subjected to two types of problems. The first is that they tend to produce variations of rigidity, and hence differential motions between different parts of the building, resulting in a local stress concentration at the notch of the re-entrant corner. The second problem is torsion. To avoid this type of damage, either provide a separation joint between two wings of buildings or tie the building strongly in the system of stress concentration and locate resistance elements to increase the tensile capacity at the re-entrant corner.
Source: https://www.routledgehandbooks.com/doi/10.1201/9781315374468-4
Size of buildings
In tall buildings with a large weight-to-base size ratio, the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be carried by columns and walls.
Clause 5.2 of NBC 204 prohibits the length to breadth ratio to be more than three. Though NBC 204 is for earthen buildings, clause 5.2 is equally applicable for other buildings as well. If the ratio exceeds 3, it is better to subdivide the building into parts keeping the ratio three or less. However, it does not mean that a building with a length to width ratio less than 3 but very large in the plan is good from a large earthquake point of view. When the plan becomes extremely large, even if symmetrical and of simple shape, the building can have trouble responding as one unit to the ground. It is because earthquakes move like waves through the earth’s crust. If the building has great horizontal dimensions, the differential arrival of the wave in different parts of the building might pose problems. [Source: NSET]
Codal Provision
Expansion Joints
IS 456:2000
Clause 27.1 and 27.2 of IS 456: 2000 provides the information for expansion joint.
27.1 Structures in which marked changes in plan dimensions take place abruptly shall be provided with expansion on joints at the section where such changes occur. Expansion joints shall be so provided that the necessary movement occurs with a minimum resistance at the joint. The structures adjacent to the joint should preferably be supported on separate columns or walls but not necessarily on separate foundations. Reinforcement shall not extend across an expansion joint and the break between the sections shall be complete.
27.2 The details as to the length of a structure where expansion joints have to be provided can be determined after taking into consideration various factors such as temperature, exposure to weather, the time and season of the laying of the concrete, etc. Normally structures exceeding 45 m in length are designed with one or more expansion joints. However, in view of the large number of factors involved in deciding the location, spacing, and nature of expansion joints, the provision of expansion joint in reinforced cement concrete structures should be left to the discretion of the designer. IS 3414 gives the design considerations, which need to be examined and provided for.
IS 3414 (1968)
Recommendations for spacing of expansion joints. [Table 2, Clause 4.4]
| S. No. | Item and Description | Spacing of Joints |
| 1. | Walls | |
| i. | Load bearing walls with cross walls at intervals. Traditional type of one-brick thick or more | 30 m intervals. |
| ii. | Walls of warehouse type construction ( without cross-walls ) | Expansion joints in walls at 30 m maximum intervals. (If the walls are panel walls between columns at not more than 9 m centres no joints are necessary.) Control joints over centre of openings may be given at half the spacing of expansion joints. |
| 2. | Chajjas, balconies and parapets | 6 to 12 m intervals |
| 3. | Roofs | |
| i. | Ordinary roof slabs of RCC protected by layers of mud phuska or other insulating media in unframed construction | 20 to 30 m intervals, and at changes in directions as in L,T,H and V shaped structures |
| ii. | Thin unprotected slabs | 15 m intervals |
| 4. | Frames | |
| Joint in structure through slab%, beams, columns, etc., dividing the building into two independent structural units | Corners of L,H,T and C shaped structures and at 30 m intervals in long uniform structures | |
| 5. | Coping | Corresponding to joints in the roof slabs |
Seismic Joints
IS 1893: 2002
Clause 7.11.3 of IS 1893: 2002 provides information on the separation between two adjacent units.
7.11.3 Two adjacent buildings or two adjacent units of the same building with separation joint in between shall be separated by a distance equal to the amount of R times the sum of the calculated story displacements as per the code each of them, to avoid damaging contact when the two units deflect towards each other. When floor levels of two similar adjacent units or buildings are at the same elevation levels, factor R in this requirement may be replaced by R/2.
IS 1893: 2016
A modification in the clause 7.11.3 has been made in IS 1893: 2016. Unlike in IS 1893: 2002, separation for building, be it in the same or different levels, shall be a distance equal to the amount of R times the sum of the calculated storey displacements.
7.11.3 Two adjacent buildings, or two adjacent units of the same building with separation joint in between shall be separated by a distance equal to the amount of R times the sum of the calculated storey displacements \Delta_1 and \Delta_2 as per the code each of them, to avoid damaging contact when the two units deflect towards each other.
When floor levels of two similar adjacent units or buildings are at the same elevation levels, the separation distance shall be calculated as (R_1\Delta_1+R_2\Delta_2) .
Source:
- https://www.aisc.org/globalassets/modern-steel/archives/2005/04/2005v04_seismic_joints.pdf
- https://law.resource.org/pub/in/bis/S03/is.3414.1968.pdf
- https://www.dudbc.gov.np/buildingcode
- https://skkatariaandsons.com/view_book.aspx?productid=8382
- https://www.linkedin.com/pulse/can-we-avoid-expansion-joints-buildings-premjit-vasudevan/