Abstract

The strain hardening fiber reinforced concrete—generally known as the Engineered Cementitious Composite (ECC)—has rapidly gained the attention of researchers in recent years. However, most of the research on ECC is limited to material and member level, leaving a gap in the understanding of its behavior at the structural scale. Therefore, this study investigates the global seismic response of ECC structures and compares their performance with conventional reinforced concrete (RC) structures. For this purpose, a case study long span building structure (an aircraft hangar having a span length of 40 m) is selected. Under the design-level gravity and lateral loads, its members are separately designed using the conventional RC and ECC. It is observed that for ECC members, the requirement of longitudinal steel is reduced by 30% when compared with the conventional RC members. Similarly, owing to an improved tensile behavior, the ECC members also exhibited a higher shear capacity than RC members, resulting in a significant reduction in the requirement of transverse reinforcement. The detailed inelastic finite element models for both design cases (RC and ECC) were subjected to the pushover analysis and nonlinear response history analysis (NLRHA) to assess their seismic performance. It is observed that (in terms of local and global seismic demands, structural damage, and ductile behavior) the performance of the ECC structure is significantly improved when compared to the conventional RC structure. The comparative cost analysis showed a reduction of 11.9% in the overall material cost of the ECC structure as compared to RC. These results show that ECC can be effectively used at the full structural level as an economic solution to ensure the ductile response and superior seismic performance.

Introduction

The structural design of long-span concrete structures is extremely challenging for designers. This is mostly owing to high moment demands, which cause undesired high tensile stresses [1] and most of the construction materials except steel are weak in tension. Despite the complexity involved in their design, they are very popular commercially. Their popularity derives from their necessity in numerous special applications such as industrial sheds, aircraft hangars, halls, auditoriums, etc. Additionally, long-span structures may assist in cutting off the cost of intermediate supports, resulting in a more economical design. In the early 10th century, these high demands were resisted by the use of arches and domes, which reduced the flexural stresses and produced axial compressive stresses [2]. Later in the 1940 s, reinforced concrete (RC) was proposed for medium to long-span structures incorporating an excessive amount of steel reinforcement [3]. Furthermore, the advancements in construction materials helped structural engineers to design cost effective infrastructures. In this way, pre-stressed concrete was introduced for long-span structures, eliminating high reinforcement and cross-sectional requirements. However, prestressed concrete possesses a shortcoming related to its ductility [4], [5]. This limitation becomes a critical concern when the structure lies in a seismically active region, as seismic design codes recommend the structure and its elements to be ductile in high seismic zones [6]. Therefore, RC with an excessive amount of steel reinforcement becomes a compulsion in seismic regions to ensure adequate confinement for ductility [4], [5]. However, RC makes the design uneconomical due to high reinforcement requirements. So, there is a need for an innovative material that can provide ductility without any compromise on cost, to ensure resilient and cost effective infrastructures.

Several innovative solutions have been proposed to improve the seismic performance of structures by providing ductile elements and links [7], [8]. One of these is to introduce fibers in the cementitious matrix to make the composite ductile. These composites are termed as fiber reinforced concrete (FRC) [9], [10], [11], [12]. A special class of high performance FRC that exhibits strain-hardening behavior in uniaxial tension is classified as Engineered Cementitious Composite (ECC) [13], [14], [15] [16], [17], [18]. ECC can be a potential candidate to reduce cross sections and reinforcement requirements due to its enhanced tensile capacity as compared to conventional concrete[19]. Additionally, ECC is lightweight and highly ductile which may also help to improve the seismic performance of the structure [20], [21], [22], [23], [24], [25], [26]. However, its material processing and design is not as simple as that of conventional concrete [27], as it requires a micro-mechanical model to obtain a mix that will ensure strain hardening response. To study its structural behavior, its material properties are incorporated by the integrated structures and materials design (ISMD) approach proposed by Li [28]. ISMD specifies that material design should be carried out first to obtain the necessary constitutive model for structural analysis and design.

Conclusions and recommendations

This research was able to conduct a comprehensive seismic assessment of the long-span ECC structure. For the very first time, both linear and nonlinear FEA models of ECC on the structural scale were developed considering a material level constitutive model. A complete structural design and nonlinear analyses of ECC structures and their comparison with RC structures were performed, and the following conclusions were made:

• Due to the lower unit weight of ECC, the design actions for structural members were approximately reduced by 25% because of the reduction in dead and seismic loads.

• Due to the better tensile capacity of ECC, the cross sectional sizes of the structural members were reduced by 25%, making ECC a good alternative to RC in the case of long-span structures. This observation indicates the potential of ECC for long-span structural applications.

• The structural design performed using JSCE guidelines showed an over 30% reduction of longitudinal steel in flexural members and about 15% reduction in longitudinal steel in compression members. Alongside, theoretically, the shear reinforcement requirement throughout the structure was eliminated, due to additional shear capacity provided by the fibers.

• The nonlinear analyses clearly showed better seismic performance (in terms of local and global seismic demands, structural damage, and ductile behavior) of ECC due to its increased capacity and lower inertial forces being developed within the structure.