Introduction

Modern civil engineering structures can achieve considerable economy in construction by combination of hybrid systems of steel and concrete or precast and cast-in-place concrete. The design of such systems takes advantage of individual material properties. The economy and speed of construction are increased also by the application of hybrid methods of construction. The main load-bearing members, formed by suspension or stay cables, hangers, 1D members or arches, are very often constructed in advance and are used as auxiliary systems for other parts of the structure to reduce overall construction time and costs. The design of the structures combines both precast and cast-in-place techniques to obtain economy in construction, and maintains a high standard of quality while reducing the time needed to complete the construction.

During the construction these structures pass through different static systems; boundary conditions change, new structural members are assembled or cast, post-tensioning is applied and temporary support elements are removed. In many structures concrete structural elements of various ages are combined and the concrete is gradually loaded. Therefore, during both construction and throughout the service life of concrete structures, account must be taken of the creep and shrinkage of concrete. Rheological properties of concrete can influence the serviceability of the structure in decisive ways. The bearing capacity of the structure can also be influenced by the redistribution of internal forces caused by creep. Therefore sophisticated methods are needed for the structural analysis.

Construction stages, Prestressing, and TDA are modules of SCIA Engineer designed for the analysis of prestressed concrete and composite structures with respect to step-by-step construction, change of boundary conditions, and rheological effects of concrete. The modules allow for the structural analysis of both prestressed concrete and composite structures, successive assembling or casting of structural elements, progressive construction of cross-sections, gradual application of loads and prestressing, and removal of temporary structural elements. Special construction technologies can be modelled, such as cantilever segmental construction with both precast and cast-in-place segments, launching, cable stayed structures, making simple 1D members continuous including successive casting of composite slab, or gradual construction of multi-storey buildings.

The implementation of these modules is the first step towards the change of design and analysis of concrete structures in SCIA Engineer. But the possibility to run the calculation in smooth sequence with respect to the step-by-step construction or the introduction of time as new variable in the analysis are not the only two aspects of the issue. Also new material parameters - rheological properties of concrete by the reduction of their E-modulus – are taken into account in the calculation and a new feature of a real value is that the program responds to modern concepts of the analysis of prestressing in theory of structures. The post-tensioned tendon is considered only as an external load at the moment of prestressing. This load is calculated as the load, which is equivalent to the effects of the tendon stressed just after short-term losses. The tendon becomes an integral part of the structure after anchoring. Its stiffness is added into the stiffness matrix of the structure. Since that, all loads carried by the structure will automatically cause the change of prestressing of that tendon. Both tendons and composite parts of cross-section are modelled by eccentric finite elements. Full strain compatibility between eccentric elements connecting two nodes is ensured along the whole length of elements. The TDA module in SCIA Engineer allows for a new structural model of so far unattainable quality.

Note: Each of the three mentioned modules can be used separately (e.g. module Prestressing in linear analysis, Construction stages for the analysis of 3D steel frame structure, etc.). However the user loses some of the features in such situation. Therefore, also the descriptions of these three modules will contain frequent links to other modules from this "little-family".

Use of post-tensioned tendons (including free tendons and cables of suspension bridges) without construction stages

When a linear calculation is performed, the stiffness of elements of post-tensioned tendons is not added to the stiffness matrix for any analysed load case. The linear calculation does not make it possible to assemble two stiffness matrices of the structure for the solution of the system, i.e. to create two left-hand sides.

Consequently, it means that we assume as if all loads (load cases) were applied in the instant of prestressing when the prestressing force is being introduced into the tendon. It means that, for example, a load case containing a uniformly distributed load applied to the main beam of a suspension bridge completely ignores the free tendons – the deck deforms and the tendons remains unstressed – the force in the tendon in the corresponding combination is equal just to the prestressing force.

The use of prestressing is thus limited to construction stages calculations and the simplification to a linear calculation may in many projects result in incorrect results.