The flow of forces through connections ? examples

The term structural connection is not limited only to the area where two (or more) elements meet-it includes the zones adjacent to both elements in which the transfer of the forces takes place. In these congested zones, the forces have to be safely transferred and the reinforcement sufficiently anchored, otherwise the connection will not work properly. The designer should draw an engineering model (scheme) of the path of the forces in the connection, for example as shown in Fig. 38, and design the joint and the adjacent zones accordingly. The strut-and-tie model is in this respect a simple and convenient tool (accurate description of the topic: ?Design of connection zones by the strut-and-tie method?).

Fig. 38. Engineering strut and tie model for column-beam connection

The designer should check that the path of the forces is uninterrupted and that the forces can be generated and resisted in every location along the path, including anchorage of tensile forces. If deformations due to shrinkage, creep, or other reasons are expected, it should be checked whether they could be allowed to freely take place. If these deformations are restrained, or partly restrained, the resulting forces have to be added to other acting forces in the connection and incorporated in the design of the connection to prevent damage or cracks. The designer should also keep in mind the influence of the execution and the workmanship on the performance, quality, and the intended flow of forces in the connection.

Three examples are given to demonstrate these principles. Fig. 39 shows a compression loaded mortar joint for the columns and a half-joint for the beams. Solution A will give an excellent performance ? the joints are easily accessible for workers and the large forces in the columns are readily transferred from one column to another. Solution B, with a beam passing over the column and one half beam joint, say at one fifth of the span, will give better moment distribution over the beams, but most probably a poorer performance at the column connection. The column connection, which now consists of two mortar joints, will have to overcome the following two handicaps :

? as the beam passes over the column and has a certain negative moment, its top filaments will elongate. This elongation transferred through the mortar joint to the underside of the upper column will negatively influence the bearing capacity of this column, as it will tend to undergo the same deformation and split,

Fig. 39. Two alternative solutions for column-beam detailing in precast concrete.
Solution A will perform better than solution B

Fig. 40 gives an example between wall elements and two hollow core floor units.

Solution A will give the best structural result. The prestressed floor units, which are normally designed for positive moments only, are free to deform and rotate at their bearings and the generally large forces in the wall elements can be directly transferred from one wall element to another.

Solution B has the following disadvantages:

o The precast prestressed hollow core slabs might be clamped in between the wall elements. Because of this the hollow core units might be exposed to negative moments,

o The lower joint will have a much smaller effective area to carry the vertical wall load due to the area taken by the supports of the hollow core slabs.

o The load from the upper wall will be forced to squeeze (écraser, comprimer) in the concrete wedge (coin) formed by the concrete fill in between the hollow core slabs. This change in the flow of the stress trajectories will cause splitting (scission, fragmentation) forces in both ends of the wall elements.

o There is a significant risk that the quality of the concrete fill in between the hollow core slabs, because of the limited space, will be less then required

Fig. 40. Two alternative details for wall-hollow core floor connection.

These two examples show how important it is to keep in mind the flow of forces through the connection and to relate this to the influence it may have on the performance of other structural elements.

The third example in Fig. 41 shows a patented** connection between precast beams and columns. The main advantage of the so-called ?sliding plate? connector is that a hidden plate inside a steel casing in the beam is pushed out and into the column unit when the beam is in its correct position. The strut and tie model in Fig. 42a shows the principal function of the vertical tie bars welded to the end plate and the vertical stirrups illustrated in Fig. 42b.

**Patent holder is SB Produksjon AS, Åndalsnes, Norway

Fig. 41. Beam-to-column connection with hidden movable steel plate

Fig. 42. Design of connection zone in the beam element:
a) strut and tie model representing the flow of forces through the connection,
b) arrangement of reinforcement in the connection in consequence of the model

Designation to the Fig. 42:

Fv - Vertical support load

Fh - Horizontal support load

T1 - Resulting tensile force at the front of the beam unit

T2 - Resulting tensile force at the rear of the beam unit

T3 - Resulting tensile force at the bottom of the beam

T4 - Tensile force caused by restraint from the column unit

C1 - Resulting compressive struts on each side of the beam unit

C2 - Resulting compressive force in the top of the beam

Date: 2016-06-12; view: 156