Part 1.2 of Eurocode 4 deals with the fire resistance of composite structures. A simple calculation method is given for assessing the fire resistance of composite beams. This paper explains the state of progress of this calculation method (in 1993) and covers both the heat transfer and mechanical response.
J.W.B. Stark and R.J. Stark, Composite structures. Analysis and design of composite steel and concrete structures for buildings according to Eurocode 4 (Steel Design 4), Bouwen met Staal, Zoetermeer 2022, ISBN 978-90-75146-06-6, formaat 23x25 cm, 228 p.
The article introduces the parts of EN 1993 (Eurocode 3) that are required when designing a steel framed building and briefly introduces EN 1994 (Eurocode 4), for composite steel and concrete structures, and EN 1992 (Eurocode 2), which covers the design of the concrete elements in composite structures.
An HSQ beam is normally smooth on the inside, i.e. it is a hollow box without either reinforcement or anything else that the concrete can grip. There is thus no mechanism for transferring the so-called shear forces between steel and concrete. Having shear connectors is a requirement according to Eurocode 4, the European standard that governs the construction of composite structures.
The SWT beam meets the requirements through the folded flanges at the top and the reinforcement at the bottom. The function is verified with tests, where the adhesion was measured as the concrete was attempted to be pushed out of the beam. It was proven that the adhesion was many times higher than the requirements.
One of the great advantages of composite elements, is that they can be designed to meet fire requirements without external protection. The concrete has good heat-insulating properties compared to steel, which means that a large part of the cross-section, except that which is directly exposed to the fire, has a lower temperature and maintains good load-bearing capacity.
For the parts that are exposed, e.g. the beam flange and the column casing, there is residual capacity in the concrete and the internal steel structures (reinforcing bars in the beam and the core in the column) that can take over loads from the heated parts. Since the total load in fire, according to Eurocode, is lower than in a cold situation, it is usually possible to achieve a high degree of optimization of the design in both cold and fire situations, through smart choices of components.
EXPERT Composite is a reliable and effective structural software solution of multistory composite and steel structures. All the features regarding the modeling, analysis and dimensioning of the structure are fully automated, providing a modern, user friendly working environment for the design of buildings made of composite structural elements.
As with other codified guidance, seismic design requirements undergo a process of continuous evolution and development. This process is usually guided by improved understanding of structural behaviour based on new research findings, coupled with the need to address issues identified from the practical application of code procedures in real engineering projects. Developments in design guidance however need to balance detailed technical advancements with the desire to maintain a level of practical stability and simplicity in codified rules. As a result, design procedures inevitably incorporate various simplifications and idealisations which can in some cases have adverse implications on the expected seismic performance and hence on the rationale and reliability of the design approaches. With a view to identifying the needs for future seismic code developments, this paper focuses on assessing the underlying approaches and main procedures adopted in the seismic design of steel and composite framed structures, with emphasis on the current European seismic design code, Eurocode 8. Codified requirements in terms of force reduction factors, ductility considerations, capacity design verifications, and connection design procedures, are examined. Various requirements that differ notably from other international seismic codes, particularly those incorporated in North American provisions, are also pointed out. The paper highlights various issues related to the seismic design of steel and composite frames that can result in uneconomical or impractical solutions, and outlines several specific seismic code development needs.
Steel and composite steel/concrete structures may be designed based on EC8 (Eurocode 8 2005) according to either non-dissipative or dissipative behaviour. The former is normally limited to areas of low seismicity or to structures of special use and importance, although it could also be applied for higher seismicity areas if vibration reduction or isolation devices are incorporated. Otherwise, the code aims to achieve economical design by employing dissipative behaviour which, apart from for special irregular or complex structures, is usually performed by assigning a structural behaviour factor to reduce the code-specified forces resulting from idealised elastic response spectra. This is carried out in conjunction with the capacity design concept which requires an appropriate determination of the capacity of the structure based on a pre-defined plastic mechanism, coupled with the provision of sufficient ductility in plastic zones and adequate over-strength factors for other regions.
This paper examines the dissipative seismic design provisions for steel and composite framed structures, which are mainly covered in Part 1 (general rules, seismic actions and rules for buildings) of Eurocode 8 (2005). General provisions in other sections of EC8 Part 1 are also referred to where relevant. Additionally, where pertinent, reference is made to US procedures for the seismic design of steel and composite structures (ASCE7 2010; AISC341 2010). The assessment focuses on the behaviour factors, ductility considerations, capacity design rules and connection design requirements stipulated in EC8. Particular issues that warrant clarification or further developments are highlighted and discussed.
The same upper limits of the reference behaviour factors specified in EC8 for steel framed structures are also employed for composite structures. This applies to composite moment resisting frames, composite concentrically braced frames and composite eccentrically braced frames. However, a number of additional composite structural systems are also specified, namely: steel or composite frames with connected infill concrete panels, reinforced concrete walls with embedded vertical steel members acting as boundary/edge elements, steel or composite coupling beams in conjunction with reinforced concrete or composite steel/concrete walls, and composite steel plate shear walls. These additional systems are beyond the scope of the discussions in this paper which focuses on typical frame configurations.
Another important consideration related to composite beams is the extent of the effective width b eff assumed for the slab, as indicated also in Fig. 5.3. EC8 includes two tables for determining the effective width. These values are based on the condition that the slab reinforcement is detailed according to the provisions of Annex C since the same background studies (Plumier et al. 1998; Doneux and Plumier 1999) were used for this purpose. The first table gives values for negative (hogging) and positive (sagging) moments for use in establishing the second moment of area for elastic analysis. These values vary from zero to 10 % of the beam span depending on the location (interior or exterior column), the direction of moment (negative or positive) and existence of transverse beams (present or not present). On the other hand, the second table in the code provides values for use in the evaluation of the plastic moment resistance. The values in this case are as high as twice those suggested for elastic analysis. They vary from zero to 20 % of the beam span depending on the location (interior or exterior column), the sign of moment (negative or positive), existence of transverse beams (present or not present), condition of seismic reinforcement, and in some cases on the width and depth of the column cross-section. Clearly, design cases other than the seismic situation would require the adoption of the effective width values stipulated in EC4. Therefore, the designer may be faced with a number of values to consider for various scenarios. Nevertheless, since the sensitivity of the results to these variations may not be significant (depending on the design check at hand), some pragmatism in using these provisions appears to be warranted. Detailed research studies (Castro et al. 2007) indicate that the effective width is mostly related to the full slab width, although it also depends on a number of other parameters such as the slab thickness, beam span and boundary conditions.
Whilst for moment frames, the dissipative zones may be steel or composite, the dissipative zones in braced frames are typically only allowed to be in steel according to EC8. In other words, the diagonal braces in concentrically braced frames, and the bending/shear links in eccentrically braced frames, should typically be designed and detailed such that they behave as steel dissipative zones. This limitation is adopted in the code as a consequence of the uncertainty associated with determining the actual capacity and ductility properties of composite steel/concrete elements in these configurations. As a result, the design of composite braced frames follows very closely those specified for steel, an issue which merits further assessment and development.
38c6e68cf9