If the columns are the legs of a building, then the beams are the shoulders. They are used to support the horizontal load to each floor, each roof and each bridge deck and they do their work pretty much silently until something happens wrong. The use of precast concrete beams for constructing that shoulder structure has become the preferable option for building in the UAE and having an understanding of where and how they are used will help the developer, contractor and consultant to make better decisions before the first column is cast.
This guide will briefly explain what a precast concrete beam is, the types of beams most commonly found in Gulf building projects and the elements in a project where precast concrete beams are most commonly found.
What is Precast Concrete Beam?
Precast concrete beam is the horizontal structural beam that is made in the precast factory and brought to the site by the hoisting machine. The beam is cast in the factory, not in the open air, therefore, it has a narrower dimensional tolerance, the same concrete strength in each unit, and in the case of prestressed beams, an internal compressive force that is created before the beam is used for the first time for service loading.
The beams produced in this way are used as the horizontal elements in the frame of a building or a bridge that bear the weight of the structure. They vary in shape and cross-section and can be manufactured by a variety of makers to meet the specific span and loading criteria, and they have demonstrated a substantial enhancement of crack resistance and load-bearing capacity when compared to an unreinforced section, thanks to the use of high tensile prestressed strands by their manufacturers such as KKK Precast.
The difference between precast and cast-in-place beam is simply a practical one. The “heavy lifting,” in all senses of the word, occurs long before the beam arrives at site in the factory. That moves labour away from a live construction site, collapses the project programme and creates a structural element that is difficult to produce in concrete that is poured into the open in the UAE hot desert, especially during the summer.
The Main Types of Precast Concrete Beams are as Follows:
It helps to understand some of the families of beams before examining applications, since the shape determines where the beam will find its place on a project. The most basic and simply named are rectangular beams which are generally reinforced or pre stressed for spanning between columns. The inverted T-beam configuration has a broad base which provides a ledge, allowing precast floor units of hollow-core slabs to rest on the beam, as opposed to against the beam thereby reducing the overall depth of the floor. L-beams are similar to ledges, but are used on one side, as they are needed along the perimeter of a floor plate where slabs need support from one direction.
The set that is used most frequently for long span work includes double-tee and i-beams. Double tees are readily fabricated in structures requiring a broad, column-free bays, like warehouses, schools, and parking decks, and are manufactured using either conventional reinforcement or prestressed and post-tensioned strands, which offer enhanced durability in repeated loading conditions. I-beams, like U-beams, are more likely to be found in bridge work where spans may extend well beyond the capacity of a rectangular or inverted T section to be used effectively.
The design of such sections is typically based on the PCI Design Handbook and the provisions of ACI 318 for precast and prestressed concrete types and is generally adapted to local loadings and environmental conditions, including testing of materials to ASTM standards. KKK Precast is not a ‘one size fits all’ shape but is made from the project’s structural drawings to fit the requirements of the structural engineer for the particular frame.
Building Frames: Where Beams Do Most of Their Work
Precast beams are most frequently used in the structural frame of the building to span between columns and support the floor slab overhead. In a precast frame, columns are placed first then beams are placed on corbels or column connections at the top of each column, and the floor slabs are placed on the beams after being locked in place. Each part is already precast on site, which means the floor elevation that would take weeks to cast and pour can often be finished in days.
It’s an ideal solution for basement and underground car park levels, where maximum depth is avoided to maximize usable car park space on a limited site, something very valuable in a market where the value of land is a key factor and headroom is of paramount importance.
Multi-Storey Car Parks
When you ask most precast engineers where they see the most use of inverted T and L-beams, the response you’ll receive varies from “parking structures” to “almost always!” On inverted T-beam, the ledge profile feature enables the hollow-core slab to rest level with the top of the beam, rather than atop it, and maintain shallow floor to floor heights, while at the same time preserving headroom for vehicles. This shallow depth directly equates to additional parking levels in a similar overall height of the building, a valuable value engineering benefit on any plot in the UAE that is limited by space.
Industrial Storehouses and Logistics Centres
The widespread expansion of e-commerce and industrial space under the UAE’s national industrial strategy has led to the preference for long span precast beams in the UAE and the Gulf region as a whole. Facility operators always prefer long spans as opposed to steel and column configurations, because one single column installed in the wrong position can affect the entire racking configuration or block a forklift aisle. Whether rebar or prestressed strand is used (based on the load), precast double tees and rectangular beams provide those clear spans without sacrificing the durability required for a 24-hour logistics operation.
Bridge Construction
Precast beam technology is still being tested at Bridges. The factory cast prestressed element is a good choice when long clear spans, high dynamic traffic loading and decades of weather exposure are desired. Precast bridge components are more than just the bridge beam: The deck slabs, girders, arches and bridge caps are all designed, and design work is usually carried out in accordance with the technical specification of the RTA as well as the PCI Bridge Design Manual, depending on the consultant’s design basis, and in compliance with bridge loading provisions found in AASHTO or Eurocode. Precast beams have been used in various types of motorway flyovers, metro viaducts and marine jetty structures, where the structural performance and installation speed are real commercial factors in the UAE.
Commercial and Office Buildings
Whether it’s an office tower, retail centre or a mixed use development, open and flexible floor plates that can evolve as tenants vary over time are exactly what a precast beam-and-slab system provides. Factory quality control and quick onsite erection time matches developer timelines in the UAE, where rental income only begins at the point of practical completion. Precast framing is valued directly, and can be easily calculated on any commercial pro forma, as it can shave 3-4 weeks off a construction programme.
Iron and Road Transport Infrastructure Projects
Precast beams are also essential for railway bridges, underpasses and station structures because they are subjected to heavy repeated loading and have a design life of decades. The use of precast concrete components during the construction of Dubai’s metro system was extensive and the uniformity created in the factory was a direct factor in the reliability the system has provided since it opened. In addition to rail, precast beams have also been used in sound barrier walls and utility structures, where their ability to resist weathering helps to reduce the cost of long-term maintenance.
They are Used in Schools, Hospitals and Institutional Buildings
Fifty years or more is a much longer timeframe on which public buildings are designed to serve a community and that time frame is more inclined to favour precast concrete. From a practical point of view, the process of erecting the structure quickly and reliably reduces disruption to an occupied campus, whether it’s a school undergoing an extension or a hospital building a new wing. Fire resistance, acoustic performance and low lifecycle maintenance are also all important considerations.
Why UAE Construction Favours Precast Beams?
The UAE is a coast country with a coastal climate which includes extremely hot summers, high humidity and salt environment, which is truly tough on concrete. The pre-cast beams produced in factory conditions also have tighter and more uniform concrete cover over the reinforcement than open site concrete production in 45 degree heat, directly resulting in increased resistance to chloride induced corrosion throughout the life of the structure. That advantage in durability is also in sync with the direction of Dubai’s Al Sa’fat green building system, and the UAE’s broader efforts to reduce the amount of resources used during construction, as factory production also results in less waste on site than traditional formwork and pour methods.
The other half of the equation is speed. It costs for every week spent on site (be those renting the plant, having to have more site supervision, or having to have a delayed hand over date), and precast framing always squeezes the construction programme compared to cast-in-place alternatives.
Final Thoughts on Precast Concrete Beams
Whether it’s an underground car park, a motorway flyover, an extension for a hospital or anything else, precast concrete beams have established their footing in most areas of UAE building. In any project, the issue isn’t so much whether precast beams should be used in the design, it’s whether they should be used in the design. It’s what beam type, span, and connection detail works best for that structure, and which manufacturer can have the engineering know-how to get the right answer. KKK Precast’s facility in Umm Al Quwain has a dedicated team of engineers who work directly from project structural drawings ensuring that precast beams and other structural elements supplied from their factory are placed as part of a larger structural system and not as individual products off the shelf.