Solving sway

Engineers know that high-rise projects pose design challenges including building sway, creep, and thermal expansion/contraction of piping systems. With the Burj Khalifa exceeding the height of any existing structure, those concerns became even more critical and high-profile.

March 16, 2010

View the full story, including all images and figures, in our monthly digital edition
Upon its completion in January 2010, the Burj Khalifa (formerly known as the Burj Dubai) in Dubai, United Arab Emirates, seized a number of records from other prominent high-rise buildings. At 2,625 ft and 160 stories, the mixed-use structure is not only the world’s tallest building, it also has broken two other significant records: tallest structure, previously held by the KVLY-TV mast in Blanchard, N.D. (2,063 ft), and tallest free-standing structure, previously held by Toronto’s CN Tower (1,815 ft). In addition, the Burj Khalifa has the highest number of stories in the world, the highest occupied floor, and the highest outdoor observation deck. Its elevator has the longest travel distance in the world and it has the tallest service elevator in the world.

Engineers know that high-rise projects pose design challenges including building sway, creep, and thermal expansion/contraction of piping systems. With the Burj Khalifa exceeding the height of any existing structure, those concerns became even more critical and high-profile.

According to William Baker, partner and senior structural engineer at Skidmore, Owings & Merrill (SOM) in Chicago, wind behavior of the structure was a top structural design consideration for the engineers designing the Burj Khalifa. Regional estimates for wind load placed the expected sway at the top of the structure at up to 6 ft overall.

The question of accommodating sway in the chilled water, domestic water, and fire protection piping systems was not just about how much the structure would move, but also about how quickly it would move given acceleration of the movement. Taking into consideration these design challenges, SOM chose to specify grooved mechanical pipe joining systems for these piping systems.

Grooved piping and deflection

Grooved mechanical pipe joining systems are most often specified as an alternative to welding of pipe and can accommodate axial movement, angular deflection, or pipe misalignment due to building creep, settlement, or thermal transients, as well as building sway due to wind loads.

Grooved mechanical couplings are available in two distinct styles: rigid and flexible. Rigid couplings are designed to fix the joint in its installed position, which eliminates any movement at the joints. Flexible couplings are designed to allow controlled linear and angular movement at each joint that can accommodate deflection and settlement.

Joints using flexible couplings allow for expansion/contraction or angular deflection within the joint. The groove in the pipe is wider than the coupling key, permitting controlled movement. The coupling key fully engages itself within the groove, creating a self-restrained joint, capable of accommodating longitudinal and lateral stresses from internal pressure and external forces. The pressure responsive gasket design provides a positive seal, with the coupling housing fully encompassing the gasket, while reinforcing and securing the seal.

Flexible grooved mechanical couplings are an alternative to welded pipe expansion means. Flexible couplings accommodate angular deflection within the design capability of the coupling and can result in space savings compared to welded loops or welded offsets.

In the Burj Khalifa, Victaulic grooved couplings were specified on a per-floor basis to handle the piping design requirements. The systems were designed to be anchored at every floor, at the top and bottom. By installing anchors at every floor, any movement of the system would be directed to the joints between the anchors. The total piping system movement, including expected building creep, settlement, thermal expansion/contraction, and angular deflection, was calculated on a per floor basis. Flexible couplings were then installed between anchors to accommodate the total movement.

Accommodating building sway

Vertical riser piping in tall buildings is often subject to deflection due to swaying caused by wind loads. Where the pipe is rigidly fixed to the building structure, freedom of motion must be designed into the piping system, allowing it to move with the building.

The anchoring of the piping systems on a per floor basis with the use of flexible couplings between anchors provided the necessary freedom to allow the piping to move with the structure.

Accommodating for building creep

Deflection or linear movement imposed on a piping system may occur due to building creep. Building creep is defined as the amount a building will move due to settlement over a specified period of time. This is an important consideration for high-rise construction and especially in the Burj Khalifa design.

Building creep can be accommodated in different ways within mechanical piping systems: using a flexible connection system, a rigid system, or a combination of both.

In a grooved system using only flexible pipe joints, risers are installed with anchors at the top and bottom with the piping being guided every other pipe length to prevent angular deflection at the joints within the piping run. A sufficient number of flexible couplings must be used to accommodate the anticipated movement. Proper gapping of the pipe ends within the coupling is required to allow the riser to compress as the building settles.

In systems with only rigid mechanical joints, risers can be treated similar to a welded system, and where movement is required, expansion joints can be designed into the riser to accommodate movement and prevent damage to system components.

Risers using both rigid and flexible grooved joints can reduce guiding requirements and accommodate the expected movement.

The bottom line on the tallest building in the world

Engineers, owners, and contractors will undoubtedly look to the Burj Khalifa as an example for future high-rise construction. Structural and piping designs that include grooved mechanical joints may alleviate challenges associated with building sway, creep, and deflection.

Grooved couplings can be beneficial to engineers and contractors, by providing design flexibility and reducing scheduling pressures and labor challenges due to the ease of installation associated with grooved mechanical joints.

Information provided by Victaulic