Joints are some times essential in buildings due to various reasons. There are different kinds of joints in a building structure. Some are needed for construction reasons,some are needed for expansion reasons and some are structuraly significant. This blog discuss the need of separation joints in building.Please ensure you read the blog till the end in addition to seeing video since some main points are mentioned in blog only
1. Expansion joint
All buildings will expand due to heat and this expansion produces tension in the building.Most buildings are made of materials that are weak in tension.The building cracks up if the expansion and the tension generated is beyond the limit of the material.This inturn results in durability issues in the building.The durability issues like rusting of rebars, if unattended can result in strength issues and failure.
There are 2 ways that this expansion can be taken care of
i) By providing expansion joints and releiving the additional stresses in the gap.
ii) By accomodating the additional stresses by additional reinforcement after doing a temperature analysis.
IS456 Clause 27.1 says a few major points as below
Expansion joint shall be provided where the buildings have an abrupt change in plan
Joints shall be preferably supported on separate columns
Joints can be on same foundation : Logic foundations are burried structures and temperature variation will not be affecting it
Reinforcement shall not extend across the joint and the break shall be complete.
Structures exceeding 45m shall preferably have more than one joint.
Code also suggests to look at IS 3414 for design considerations.This code gives elaborate guidelines on location of expansion joint,materials to be uesed how to apply these meaterials and so on. The tricky point is that this code recommends far lesser spacing than the 45m suggested by IS 456.
Generally clients and architects does not like the idea of expansion joint in buildings as it becomes a succeptible point for leakage. IS 3414 provisions do not help this situation either. In general practice,I have observed that even more than 50m dimension do not produce any large expansion and cracking is rarely observed. So it may be wise to use the discretionary provisions in IS 456. IS 456 27.2 says ''However inview 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.
I am of the belief that even discretion needs 100% justification. Hence it would be a good idea to do a temperature load analysis and accomodate the additional stresses if any. Software like etabs and staad pro can do a temperature analysis and calculate this additional requirement. Civilera has a course on temperature analysis in the etabs tutorials.
2. Structural Geometric Irregularity
Though IS 456 in clause 27.1 recommends expansion joint where there is abrupt change in plan, this joint requirement due to plan change is more relevant for making a building perform better in seismic eventualities. What I mean is this joint requirement is more relevant for making the building regular than the expansion reason. This is more explained in the Video.
There are 4 virtues that control the seismic performance of buildings and a regular geometry is one among them.
3. Mass Irregularity
There can be situations where there are more floors on one part of the building and less in another part.This can create 2 issues
a) Differential settlement of foundations.
b) Mass irregularity which can inturn induce torsion in the building.
In this case also it is better to induce a separation joint.
4. Addition to existing structure
Here too, it is ideal to separate. The soil under existing structure would be pre consolidated due to the loads. Also the seismic demand of the building would change and hence it would be difficult to meet the requirement.
Should the joint be provided in footings
As the foundation of a building is not exposed to atmosphere,the temperature changes will not affect it. In most cases the foundations are burried in soil. Therefore it is not needed to provide joints in foundations if the reason for the joint provision is expansion. If there are considerable load difference in different portion of the building, it may help to separate even the footings as it can be made economical some times by separation. This needs to be looked in to on a project to project basis.
What should be the gap width?
The expansion gap width in a tall building is not generally decided by the thermal expansion.The seismic reason also need to be considered. When we separate the buildings, there is a chance of pounding of the two buildings if the deflections are excessive. This needs to be taken care as per IS 1893 :2016. Based on the Resonse Reduction factor,R, the joint width needs to be decided.
Also this gap need to be covered so that the building remain leak prrof. All these points are mentioned in detail in this forum section here.
Also read my short article on linkedin here.
Summary
It is important to know that all separation joints are not for expansion reasons.
The gap for an expansion joint can be decided based on thermal properties criteria and as per the provisions in IS 3414. The expansion joints can be eliminated if one absorbs it by doing a temperature load analysis.However in many large buildings, the need of the joint is due to structural reasons and these joints cannot be easily avoided unless torsion is accomodated by design which is not the best solution to provide.