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In this article:
Hambro, a business unit of Canam Group, designs, manufactures and markets a concrete floor system used primarily in multi-residential construction.
:: The Hambro system
Hambro’s composite floor system combines steel joists and poured-in-place concrete. It consists of concrete-steel T-beams running in one direction and an integrated continuous slab in the other.
Once the concrete is poured, the top chord is embedded in the concrete to produce the shear required between the steel and the concrete. The concrete slab is reinforced with welded wire mesh draped over the top chord. The special S-shaped top chord functions as a high chair, creating negative and positive moment capacity in the concrete, which behaves as a continuous one-way reinforced slab.
The ends (shoes) of the Hambro joist are completely embedded in concrete and are connected to the principal member (Hambro girder or other support).
Figure 1
:: New product
Hambro has added a new composite “concrete-steel” girder, intended for the multi-residential and commercial markets, to its existing range of products. Known as the “Hambro Girder”, it acts as a principal beam supporting the Hambro joists installed perpendicularly at regular intervals on each side. The advantage of the new composite girder is that it offers greater spans than the conventional steel girder while maintaining a minimum depth to adapt to the Hambro joists. A major development with this new concept is the shop installation of all concrete-steel connectors to eliminate the tripping hazards associated with the field installation of conventional metal studs. The fact that the Hambro shoes are field-welded to the girder also eliminates exposure to the tripping hazard addressed in the OSHA standard. The concept was patented in Canada and is described under “Shear connector characteristics” below.
Figure 2
:: Girder characteristics
The composite Hambro girder is composed of a top and bottom chord, each made of two angle sections. The web system consists of vertical and diagonal members. The top chord S connector closes the opening between the two angle sections. In addition to the S connector and where necessary for shear requirements, U-shaped channels developed specially by Hambro can be shop-installed on the girder between the Hambro joists.
:: Shear connector characteristics
The longitudinal shear that occurs between the concrete slab and the steel girder is due to the action of the following four elements:
- the joist shoes welded at regular intervals to the girder. This concept, patented under the name “Shear Shoe”, utilizes a secondary structural element (joist) to create the longitudinal shear of a primary structural element (girder).
- the anchorage created by the end plate welded at each end of the girder. The horizontal force is associated with the resistance of the concrete.
- the bond friction that occurs between the joist top chord S and the concrete slab.
- the additional connectors used to support heavy loads (U-shaped steel channels welded to the girder at regular intervals). These U-shaped channels were developed in collaboration with the Centre de recherche industrielle du Québec (CRIQ) and are always installed in pairs (one welded on the top of each angle of the top chord member).
These four methods of creating shear action between the steel and the concrete slab comply with the OSHA standard since the concrete-steel connectors do not pose a tripping hazard for workers walking on the girder.
:: Advantages of the “Hambro Girder”
- Composite action between the slab and the girder
Utilizing a concrete slab in the overall design of the Hambro girder allows a better span-to-depth ratio than that of a conventional girder. Although a ratio between 24 and 27 generally represents an efficient and economical solution, a composite product can reach a ratio of up to 30.
For example, in the case of a 9.14-metre (30-foot) girder, the optimum depth is approximately 46.93 centimetres (14 inches), while the minimum depth is 30 centimetres (12 inches).
Due to the composite design of the Hambro girder, the top and bottom chord members are smaller than those of a conventional girder. This provides for wider openings between the members.
The vertical members of the Hambro girder are aligned with the joists bearing on the top chord, maximizing the size of the openings between the diagonal members and the bottom chord.
Figure 3
- Fire resistance rating tested to ULC and UL standards
Composite Hambro girders form an integral part of the composite Hambro floor system and therefore can reach a fire resistance rating of up to 2 hours, depending on the type of ceiling used.
- Reduced number of columns
Composite Hambro girders are ideal for floors requiring long spans since fewer columns are utilized.
The optimum span of a composite Hambro girder ranges between 7.62 and 9.14 metres (25 and 35 feet).
- Camber designed for the dead load of wet concrete
Hambro girders and joists, designed with a camber according to the dead load of wet concrete, become level after the pour. This provides the flexibility to install the ceiling directly beneath the bottom chord in accordance with ULC and UL specifications.
- Ability to support extra heavy loads
With a concrete slab and steel members incorporated into its design, the composite Hambro girder can carry extra heavy loads such as the upper floors of a building.
:: Technical questions
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