Built Expressions Bangalore :: Expansion Joinnt Treatment in Building Roofs

Comments (0)

Comments This Article

Expansion Joint Treatment in Building Roofs

Bureau Built Expressions

An expansion joint is a separation that appears through the entire construction including floors, walls, façades and roofs. These joints absorb building movements to control and prevent cracking. They also provide separation between different structural elements, including walls, machinery foundations, drains and so on, avoiding the transmission of stresses induced during the movements. Not providing expansion joints and using some unsuitable joint system for distributing stresses induced can encourage irreparable damage to the structure.

Instead, expansion joints prevent transfer of stresses so that each of the blocks operates independently and does not knock against the block next to it. The placement and opening of these joints are determined by the estimator or structural engineers, who will also determine the load capacity that the joint must bear.

Subsequently, these expansion joints are treated and covered using appropriate materials like joint filler boards, joint cover profiles. Expansion joint profiles consist of metal profiles (aluminium, steel, stainless steel, etc.), generally joined by rubber inserts, and covering the openings to allow movement in the blocks without causing damage to the other structural components.

IS: 3414 - 1968

As per IS: 3414-1968, Expansion Joints are defined as - "Joints 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 ability to regain 75 percent of the original thickness, when pressure is released.”

The above code recommends the expansion joints used in roofs shall be finished such as to obtain an effective seal against penetration of water. A water-bar shall be installed in the expansion joint. The joint and the cover slabs shall be suitably treated for waterproofing.

Why Expansion Joints

The key to successful building design is recognizing that while the individual members in a structure may only experience small changes in length, the cumulative effects of these changes can become significant and should always be considered. This is especially important when looking at buildings with a large horizontal dimension. Expansion joints that separate the entire structure and roofing membrane will allow for the release of internal stresses that would otherwise build up. This will help to ensure that relatively brittle cladding, particularly masonry, is not subjected to excessive displacements and consequential separation or cracking, which could allow water to penetrate into interior spaces.

For example, when a structure heats up, the building materials from which it is built expand. This causes the "expansion joint" to close down, thereby compressing the expansion joint system installed in the gap. Conversely, when the temperature drops, the materials cool causing the joint gap to open. This requires the expansion joint material to expand to follow the joint movement.

Movement Causes

Parameters that can cause movements are listed as under;

Variations in temperature of the environment.
Expansions generated from the contraction of the different construction materials.
Settlement and movement of the terrain
Seismic movement
The weight of the buildings themselves and their use.
Wind pressure.
Loads and traffic.

Classification

Expansion Joints are classified based on the methods of installation, the locations they appear, load they support and the movement capacity.

According To Installation: Overlaid on the floor and Built-in flush with the floor.
According To The Load They Support: Light, Medium, Heavy and Very heavy.
According To Purpose: Floor-floor, Wall-wall and Floor-wall.
According to their movement capacity: Thermal joints and Seismic Joints.
1. Ø Thermal joints: are those that admit the expansion of the construction elements themselves. Habitually these joints do not surpass 50 mm joint widths and expansions of +/- 10 mm (0.39”)
2. Ø Seismic joints: as a general rule, seismic joints are classed as those that have a multi-directional expansion apacity of at least 25% of the width of the joint in each horizontal direction. For example, in the case of a joint 50 mm wide, in order to class it as a joint with seismic capacity, it must have an expansion of at least +/- 12.5 mm.

Design Factors²

There are too many variables when looking at issues of expansion and contraction to create a simple set of rules that govern design. This is ultimately a serviceability issue that must be left to the judgment of a qualified engineer. If the components that are subjected to the movements of thermal expansion are highly sensitive to differential displacements, the tolerances will be much lower when considering maximum elongation values.

This may require the designer to place expansion joints at a closer spacing than would be provided for a similar building with less stringent movement requirements. Additionally, there may be other factors like creep, shrinkage, yielding, or swelling of members that contribute to the displacements which are allowed for in the design of expansion joints. The governing design condition may occur during construction and be less severe when the structure is enclosed and in operation.

One guideline to consider when designing for expansion is to allow the structure to move as a whole as much as possible and to keep the differential movement of components to a minimum. It is also important to ensure that the design allows for sufficient load reversal to account for the effects of contraction as well as expansion of components. There may be components of the expansion or contraction that are the result of permanent deformations. It is necessary to account for this type of behaviour because it may require that the allowable slip in a joint be greater in one direction that the opposite direction.

In the end, the internal stresses generated by thermal loads or other sources will find a mechanism for release. If there is not a well thought out path or design component to control this stress release, an unintended crack will develop somewhere in the structure. If that is the intended mechanism for the release of internal stresses, then designers are advised to provide a mechanism to control the widths of the cracks that will develop. Without providing expansion joints or controlling crack widths, the structure will likely suffer the effects of water infiltration and the secondary effects of corrosion and other adverse conditions.

Roof Joint Treatment

The process of treating the roof expansion joint actually begins much earlier. The gap between the two blocks is gradually filled using filler boards which form an important component. These filler boards are generally bitumen impregnated. Typical raised roof and Levelled roof expansion joints are shown in the figures.

Filler Boards- Bitumen Impregnated Expansion Joint Filler Boards display excellent resistance to compression, with outstanding recovery characteristics. Provision of these boards in the joints permit free movement and also prevents damage to the slabs besides providing waterproofing and resisting entry of foreign matter into the joints. It is a low density high quality fibre board which is cellular in nature, securely bonded together and uniformly saturated with Bituminous Compound. Impregnation with Bituminous Compound makes it durable and water proof. Cellular nature provides compressible properties and ability to recover after compression load is released without breaking or cracking. It ensures sealing of joints as no gap exists between the joint spaces. There are several other types of fillers that are available in the market.

General procedure for the expansion joint treatment in roof involves, cleaning the top surface of the filler boards, proving the back-rods to avoid gaps and seal the joint with the specified sealant using sealant guns. Subsequent to this, the top of the joint is covered with protection strip that allows for the movement. These joints are available in the market with different materials, shapes and colours.

References: