Examination of design rules for high ductile steel buildings with composite column moment frames

Reinforced concrete and structural steel, the two most preferred structural systems in building design, have advantages over each other. While steel stands out with properties such as high strength, high ductility, low margin of error in material production and ease of construction, reinforced concrete provides advantages to the designer with its affordability and resistance to the effects of pressure. It is possible to make use of the advantages of both systems by using composite structural systems. The composite use of steel and concrete in structures was seen for the first time in 1894 in the United States. The importance of composite members was understood after the 1906 San Francisco Earthquake and the subsequent fires, when steel structures encased in concrete or brick performed better than all other structural systems used at the time. Even though the use of composite columns and beams in multi-story structures increased following this event, the effect of composite action on strength and stiffness was overlooked for a long time and only its contribution to fire safety was taken into consideration. Afterwards, composite structural systems started to be used more often as steel composite members were included in developed countries' regulations. Today, they continue to be frequently preferred due to the advantages they provide. Within the scope of this study, firstly composite structural systems are analyzed under the categories of composite members and composite structural systems. Composite members are divided into composite slabs, composite beams, composite columns, composite braces and composite shear walls. The characteristics of these structural members are briefly reviewed, and then composite structural systems are analyzed under the two main categories of composite moment frame systems and composite braced frame systems. In the next section, the development of composite structural design in the United States is discussed. The changes over time in Specifications for Structural Steel Buildings and Seismic Provisions for Structural Steel Buildings are evaluated in terms of composite members. Then, the rules of design provided in the composite members section of the 2016 regulation Principles of the Design, Calculation and Construction of Steel Structures are examined in detail for composite columns, and this section is compared against the current regulation in the USA. Afterwards, the special rules of design for steel structures with composite columns specified in Turkish Earthquake Code for Buildings 2018 are described. After detailed examination of the regulations, an example building of high ductile moment frames with composite columns is analyzed according to Principles of the Design, Calculation and Construction of Steel Structures 2016 and Turkish Earthquake Code for Buildings 2018, and the results are evaluated. Concrete filled composite columns are used in the example structure. ETABS 2016 v16.0.3 software is used for xxviii structural analysis; effective length method and direct analysis method are applied separately as the method of design. Before proceeding with analysis of the example building, the software's handling of stiffness reduction and second order effects, which are the basic principles of direct analysis method, is examined on a simple frame system and the results are presented in tabular format. The loads which will apply to the structure are identified according to the relevant regulations, and the horizontal seismic load effects are applied usingmode superposition method and equivalent static load method separately. After structural analysis, displacement and equivalent static seismic load checks are performed according to Turkish Earthquake Code 2018, and then strength checks of the secondary beams, moment frame beams and composite columns are performed relative to the internal forces obtained from the analysis performed for each method of design. After strength checks, the results obtained are evaluated. It is observed during structural design that the essential factor determining dimensions of structural members is relative story displacements. The steel moment frame system displays quite flexible behavior, which causes displacement criteria to be the determining factor rather than strength criteria. Strength checks also yield quite close capacity ratios for the 4 design types, and it is observed that required strengths are fairly below capacities. The study covers an investigation of the compatibility of the two methods of seismic analysis which can be used in the analysis of a high ductile steel building with composite column moment frames, equivalent static seismic load method and modal seismic load method, with the two methods used in analysis of the building, effective length method and direct analysis method. The results show that for both methods of design, analysis with equivalent static seismic load (if Turkish Earthquake Code 2018 allows its use) yields consistent results. When modal seismic loads are used, it is observed that the seismic loads which apply to the building are reduced along with a reduction in the rigidity of the structure due to the second order analysis performed on ETABS with preset gravity loads to account for the impact of second order effects on internal forces. Nevertheless, internal forces are increased as expected due to second order effects. When stiffness is reduced as per the requirements of direct analysis method and modal analysis is performed on this reduced-stiffness system according to second order theory, the structure's stiffness is considerably reduced and this leads to a significant decrease in modal seismic loads. By extension, direct analysis method, which is employed to account for non-linear deformations and stiffness uncertainties of materials, significantly reduces internal forces whereas it was expected to increase them. The main reason for this is that the τb coefficient to reduce flexural stiffness in composite columns is taken as 0.8 regardless of axial load. But when modal seismic loads together with direct analysis method are used with amplified first order analysis, more reasonable results are obtained as there is no decrease in modal seismic loads. Based on this result, it is found acceptable for direct analysis method to be used together with modal seismic loads in steel structures with composite columns when amplified first order analysis is employed.