Rigid joints in precast concrete – a new generation of Colmshoe connectors

The new family of Colmshoe connectors is a response to the growing needs of modern prefabricated construction. It is an extended range of column supports with faster and more reliable assembly, flexibility of applications and safety confirmed by the National Technical Assessment ITB.

Precast concrete construction in industrial and public buildings

Precast concrete products are commonly used in industrial construction and for the erection of large-scale public buildings or warehouses [1]. Interest in prefabrication will undoubtedly grow due to sustainability requirements, advances in building materials engineering and a shortage of skilled workers on construction sites. The effective use of high-strength and ultra-high-performance concretes is possible with the appropriate technological regime, which is guaranteed by the production of elements at the prefabrication plant, and the connections of elements made of such concretes require thoughtful and safe solutions [2].

Rigid and hinged joints in prefabricated structures

In a prefabricated load-bearing structure, it is possible to distinguish between structural elements (beams, columns) and the joints between them (hinged, semi-rigid or rigid). A typical example of a rigid joint, transferring forces and moments, is the connection of the column to the foundation. To make this type of connection, starter bars anchored in holes formed by corrugated pipes are used, but more often – pre-stressed bolted connections, consisting of foundation anchors and column supports (Columnshoe), see Figure 1.

Fig. 1. Bolted foundation-column connection

Advantages of bolted connections

The main advantages of bolted connections include:

• shorter assembly time of columns at the construction site (increase in processing capacity by up to 100%),

• efficient connection immediately after screwing the connectors (no need to use temporary supports),

• greater reliability of the connection due to the precision of assembly and precise verification of the performance properties (in particular load-bearing capacity and fire resistance) of the system connectors.

Advantages of the new generation of Colmshoe supports from PFEIFER

Modern methods of static-strength analysis were used in the design of the new series of column connectors. This made it possible at the outset to efficiently design their cross sections affecting the economy of materials used in production. The obtained analytical parameters of the fastener’s load-bearing capacity were confirmed by appropriate strength tests.

Colmshoe is made up of two groups of connectors (see Fig. 2):

• CoRni corner supports

• SiDi side supports

More about the new supports: 👉 New Colmshoe Column Support Family

Each of the Colmshoe groups includes nine types of supports with different sizes and tensile and compressive capacities designated by the capacities of the individual elements from 70 to 530 kN. For example, corner supports can be distinguished by the size CoRni 70, and for side supports SiDi 70 (supports with a load capacity of 70 kN).

Fig. 2 New generation of Colmshoe column supports

The innovative shape of SiDi connectors eliminates eccentricities

The new generation of corner supports came about as a result of evolutionary changes made after a more thorough static-strength analysis of this group of column supports.

Particularly noteworthy, however, are the SiDi side supports – in which a completely new section of the connector, not yet used in the construction market, was used. The solution used is ground breaking – it was possible to obtain a connector that virtually eliminates the eccentricity of anchoring the base of the column support with respect to the anchor bars of the connector.

Both groups of supports can be used separately or in a mixed system, see Figure 3.

Fig. 3. Examples of the arrangement of side and corner supports in the cross-section of a prefabricated column

The development of a new group of lateral supports was possible thanks to a thorough understanding of the static work of this construction detail. The eccentricity between the axes of the foundation anchor and the reinforcing bars causes large bending moments in the base and the rib of the support and the formation of a couple force, which in the case of compression of the support is transferred by the pressure stresses to the concrete and the rear bar, see Fig. 4. In the case of side supports, this eccentricity does not occur, which allowed for a reduction in the thickness of the sheets and the elimination of the rear bar. A small eccentricity can only occur due to the deviation of the anchor axis within the assembly tolerance. However, the values of moments and horizontal forces caused by this eccentric are small, and the stability of the support in the horizontal direction is ensured by shaping the rib into a characteristic boomerang shape.

Fig. 4 Simplified calculation scheme of Colmshoe support in compression

Modern FEM analysis methods in the design of connectors

Another aspect that enabled the optimization of column supports was the implementation of modern methods of static-strength analysis based on three-dimensional Finite Elementary Method (FEM) models. The FEM analyses took into account: the elastic-plastic material model of the steel, the contact between the base plate and the anchor washer and the possibility of the rib breaking away from the concrete mass. Example FEM models for a size 100 support are shown in Fig. 5. The nonlinear FEM analysis resulted in the force-deflection curves of the base and maps of reduced stresses and plastic strains, which allowed for an accurate estimation of the load-bearing capacity of each support. The correctness of the obtained results was verified by appropriate destructive testing.

Fig. 5. FEM model of the corner support and the side support

The impact of modern technologies on reducing the carbon footprint

This article presents a possible solution for a rigid connection of a column to a foundation using bolted connectors, using the new generation of PFEIFER Colmshoe column supports as an example. The design process of this product illustrates how modern analysis methods, previously commonly used in the automotive and aviation industries, enable the optimization of connectors for precast concrete elements. Due to the industrial nature of their production, this approach is cost-effective and significantly contributes to reducing the carbon footprint of new buildings and structures constructed using prefabricated technology.

Bibliography

[1] G. Adamczewski, P. Woyciechowski: Prefabrykacja – jakość, trwałość, różnorodność. Zeszyt 1. Stowarzyszenie Producentów Betonów, Warsaw 2014.

[2] K. Janczura: Kierunki dalszego rozwoju konstrukcji z betonów prefabrykowanych – od B15 do C 110/115. Dni Betonu 2014, Wisła 13-15 October 2014.