Sky bridges linking between two (or more) buildings at height has become popular for decades. These link bridges are not only serving for people circulation from one building to another, but also the key aesthetic feature and function in the architectural design for several iconic buildings.
Behind those attractive sky bridges, there are several challenges to the structural engineering design which need to be properly addressed. Structural concepts of these bridges are crucial. Incorrect structural concept design can potentially lead to some failures or even complete collapse of the bridges and overall building system especially under extreme loading events. This article aims to provide information for local structural engineering community on some key design parameters and to raise awareness on structural design challenges of the sky bridges which are now becoming more popular in Thailand. It shall also be beneficial for the asset owners to investigate on their existing link bridges for potential risks and/or concerns.
Types of Sky bridges
Although sky bridges serve as a key architectural element for aesthetic, people circulation, and other functions, their types classified from a structural engineering point of view can be different.
Depending on the criteria, there can be several types of sky bridges. For the context of this article, sky bridges will be classified based on the stability requirements, which may be grouped into two major types as follows:
Stability elements
These link bridges classified as stability elements are structurally required for overall stabilities of the whole building system. Without these bridges, each individual tower may not be stable or hardly to economically achieve stability requirements. An example for this type of link bridges is the CCTV Headquarters building in Beijing, China designed by ARUP.
These bridges will need to be designed to withstand the relevant design loads and relative movements; in any directions, between the linked buildings under various loading conditions.
Non-stability elements
This type is the most common structural types of sky bridges in Thailand. The non-stability link bridges are considered as isolated elements connecting between buildings. Their presences are not structurally required for the building stability as those buildings can withstand relevant loads without the bridges. Although they are considered as isolated structures, their failures can be fatal. An example for this non-stability sky bridge is a link bridge connecting the Petronas Twin Towers, Malaysia designed by Thornton Tomasetti.
Key Design Parameters
Strength and stiffness
Basic requirements for both types of link bridges are to have sufficient strengths to resist all required loads such as gravity loads, winds, earthquakes, thermal, etc. While the non-stability sky bridge will mainly require self-strengths to withstand the imposed loads, the stability type is more complex. Its interaction with the supporting buildings is required to be considered in the analyses to achieve the overall stability requirements, e.g. lateral acceleration, lateral sway, etc.
Similar to strength requirements, stiffness of the sky bridges will need to be proportioned to suit their purposes; under both serviceability and ultimate limit states, whether they are stability or non-stability types. As one may anticipated, dimensions of the stability-type bridge will normally be much larger compared to the non-stability type due to higher demands on both strength and stiffness.
Relative movements
Visualize a link bridge as a small rope connecting two ships floating in the rough ocean, one will easily understand why relative movement will be such an important issue to consider.
"... even the twin buildings will never behave the same."
Both types of sky bridges will experience significant relative movements; both vertical and horizontal directions, between their supporting buildings. Predicting movements of the supporting buildings can be very challenging as there are huge unknowns which may not be able to accurately predict such as time-dependent material properties, actual constructed conditions of the structures, actual loads imposed on the buildings, etc. Several tests/experiments and field adjustments are anticipated. It shall be kept in mind that even the twin buildings will never behave the same.
"... it is unlikely that the slotted-bolted connection details will be suitable for neither the required displacement magnitudes nor movement directions."
The higher the bridge’s linked location, the larger the relative displacement to be experienced by the link bridges. Connection details between the sky bridges and the supporting towers will need to accommodate these movements which can create significantly high stresses in those connections especially for the stability-type bridges. For non-stability type bridges, it is common to use sliding bearing connection details between the link bridges and the supporting buildings, however it is unlikely that the slotted-bolted connection details will be suitable for neither the required displacement magnitudes nor movement directions. Failures of these supporting connections can be catastrophic.
Measures to prevent unseating of the non-stability bridges under extreme loads (e.g. seismic) are also required to be properly considered in the design. Poundings between the supporting buildings and the non-stability bridges at their interfaces are also crucial and need to be prevented.
Construction methods
Sky bridges are normally linked at high level above ground. Construction method of the bridge can be one of the governing design criteria especially on materials and structural system which shall normally be light-weight and can be prefabricated. In-situ reinforced concrete structures are rare for the sky bridges as it is normally uneconomical or impossible for temporary props on the required formworks and also the heavier mass compared to other materials, however, it may still be applicable for sky bridges linking at low level.
"Construction method of the bridge can be one of the governing design criteria ..."
There are several requirements from a construction point of view which need to be considered in the design stage such as lifting VS cantilever methods, capacity of the lifting equipment, construction sequences, access of construction equipment on ground floor, assembly area for the prefabricated segments, etc. These will have some certain impacts to both the bridge structures themselves and their connection details with the supporting buildings.
Sky bridges are important for both aesthetic and their required functions. There are several structural design challenges which need to be considered thoroughly, otherwise they could pose high risks in life and safety to large numbers of people, especially under the extreme loading conditions.
Are your sky bridges the weakest link in your buildings?
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