Composite Steel and Concrete Structures: Technology and Design

Composite Steel and Concrete Structures: Technology and Design Edoardo Cosenza1, Luigi Di Sarno2, Giovanni Fabbrocino3 and Marisa Pecce2 1

Department of Structural Analysis and Design - University of Naples Federico II 2 Department of Engineering - University of Sannio, Benevento 3 Department of SAVA, University of Molise, Campobasso

Abstract: The present paper reviews the fundamental structural response characteristics and technological issues of composite steel and concrete systems. It assesses the pros and cons of composite structural systems and investigates the efficacy of beam-column members. Design rules for composite constructions are presented and discussed in details in order to get deep insight into their background. Areas of further research are presented and results of ongoing experimental and numerical investigations are also shown.

1

Introduction

Composite constructions include a large variety of structural systems, e.g. framed structures employing all composite members and components (composite beam-columns and joints) and sub-assemblages of steel and/or reinforced concrete (RC) elements. Such components and/or elements are employed to optimize the resistance and deformation capacity (Uchida and Tohki, 1997). Hereafter, composite columns and framed systems are discussed in details both from a design and technological standpoint. Composite steel and concrete columns are relatively recent components used in framed structures. Analytical and experimental tests on these members have been developed in the last decades and the development of design rules is still ongoing (Elnashai et al., 1990; Cosenza et al., 1998). The advantages in using composite columns are, however, evident (e.g. Cosenza and Zandonini, 1997, among many others). For example, the Millennium Tower, which was a challenging design project due to the analyses performed along with the employed detailing. The building uses slim-floor as decks; the columns are concrete-filled and their connection with the floor deck is semi-rigid (Huber, 2001). Recently, the feasibility of using composite constructions in seismic areas has been investigated and several types of joints, either traditional or innovative, and members have been produced via prefabrication. Notwithstanding, the applications of structures with all composite members are limited. Commonly, horizontal actions, e.g. wind and/or earthquakes, are resisted by RC walls and/or cores, while bare steel systems carry the vertical (gravity) loads. The seismic response of composite structures is generally not straightforward: local and global effects may interact and unexpected failure modes occur. It is therefore of paramount importance for the assessment of composite structures to account for local interactions, e.g. interface behaviour between steel and concrete, and global behaviour, e.g. joint response, both beam-to-column and base-column. Adequate local and global ductility is essential to ensure stable hysteretic behaviour of the system under earthquake ground motion. Hereafter, the fundamental response characteristics of composite structures are reviewed along with existing national and international design provisions. The performance of composite structures also discussed with regard to framed systems.

2

Composite columns

2.1 Types and structural performance Composite columns include the following (see Figure 1): -

Fully encased: steel members with cross-sections fully encased in concrete; Partially encased: steel members with cross-sections partially covered by concrete; Concrete-filled: steel hollow sections filled by concrete. Concrete, either reinforced or plane, prevents the onset of local buckling phenomena (Boyd et al., 1995). The latter typically affect bare steel structures and erode their stiffness, resistance and ductility. Local buckling is inhibited in fully encased members, while its occurrence is minimised in partially encased beam-columns. Additionally, steel confines concrete thus augmenting the compressive resistance, particularly in concrete-filled columns. The increase in resistance is generally limited for rectangular cross-sections, but is significant for circular sections. The concrete shell ensures fire and corrosion resistance, especially for fully-encased members in which the degradation of mechanical properties with high temperatures can be lowered (Cosenza et al., 1994; Nigro et al., 1998). From the technological standpoint, composite columns generally exhibit easy of construction as they do not require formworks. Indeed, for concrete-filled members, concrete is cast within the steel shell, while in partially encased components, the cast is carried out horizontally and the element rotated to fill-up each side. Partially encased elements can be thus prefabricated in the workshop and completed on site by using special filler and/or joint devices. In composite beam-columns, the detailing is of paramount importance. It may vary significantly as a function of the cross-section and the type of composite members. Quality of concrete cast, shrinkage, loading condition, e.g. static and/or cyclic, and load transferring affect structural response of composite beam-columns. cy

bc b

cy

b=bc

a

b

cz

h hc y

h= hc

y

tw

tw

tf

tf

cz

Z

Z

e

d

d

b

t

t

h

Y

y

t z

Z

Figure 1. – Typical layouts of composite steel and concrete columns (after Eurocode 4, 1992).

Composite columns are commonly employed in modern medium-to-high rise buildings (Ricles and Paboojian, 1994). These elements combine the strength and lightness of steel

construction with stiffness and economy of RC systems. In several modern tall buildings, composite columns are used in the lower floors while bare steel columns are employed in the upper parts of the structure. As a consequence, the size of cross-sections can be optimized height-wise and the construction time reduced, as the steel skeleton, made of bare steel members and decking, is arranged first and then concrete is cast. To exhibit efficient structural response, composite columns should employ shear connectors along the height. However, the presence of these connectors may affect detrimentally the quality of concrete cast and generally increases the cost of construction. Several experimental tests showed that shear connectors can often be omitted without eroding the structural performance (Roeder, 1998). In earthquake-resistant structures, composite columns are used in hybrid systems, in which lateral loads are carried by RC walls and/or cores, or bracing systems, made of either of bare steel or composite. To-date, several experimental tests have been carried out on partially and fully encased columns, while less data are still available for concrete-filled members (Shanmugam and Lakshmi, 2001). Partially encased columns were tested both cyclic and pseudo-dynamically (Elnashai et al., 1991; Ricles and Paboojian, 1994; Elghazouli and Dowling, 1992). The structural response is characterised by reduced buckling, both local and global, as well as low sensitivity to second-order P-Δ effects (Elnashai and Elghazouli, 1993-a,b). Moreover, composite columns possess excellent ductility due also to the high concrete confinement (Hajjar, 2002). Recent experimental tests carried out by Varma et al. (2002) demonstrate that cyclic loads do not affect significantly the stiffness and strength of concrete-filled members. The results are, however, a function of the level of axial load, the slenderness ratio of the wall thickness and the steel grade. Extensive experimental and numerical tests are still ongoing, especially in Europe (e.g. Plumier and Doneux, 2001; Bursi et al., 2004). Such tests are aimed at improving the reliability of existing design rules and get deeper understanding of seismic response of composite sub-assemblages, components and their joints, especially beam-tocolumn and base-column. 2.2 Structural assessment and experimental tests To assess the structural response of composite steel and concrete systems, two levels of modelling should be considered: these include modelling at section-level and memberlevel. In the former case, the assumptions adopted are those typical of RC components, namely strain compatibility between steel and concrete, zero-tensile strength for concrete and preservation of the planeity of the cross-section. Comparisons between analytical and numerical results confirm the reliability of using stress-blocks for section analyses and designs. Refined modelling to account for the confinement effects in concrete-filled members were carried out by (Pecce, 1993), while comparisons between different methods of analysis and design can be found in Nigro (1995). On the other hand, modelling at member-level should account for effects of geometric and mechanical (e.g. Basu and Sommerville, 1969) nonlinearities. To-date, there is a paucity of data with regard to postelastic and cyclic behaviour of composite columns. Fiber and hysteretic models generally used for structural assessments (e.g. Ricles and Paboojian 1994; Elghazouli and Dowling, 1992; Mirza et al. 1996; Fujinaga et al., 2000) require further developments. Recently, Alav et al. (2002) suggested a simplified non-linear model to assess beam-columns under large displacements. A comprehensive state-of-the-art of the analytical models and experimental tests can be found in Shams and Saadeghvaziri (1997). Reference codes of practice for steel and composite structures, e.g. buildings and bridges, in Europe is Eurocode 4 (1994, 1997). Recently, Italian recommendations named CNR 10016 (1999) were also implemented; they propose design rules for static loads in compliance with Eurocode 4. Conversely, Eurocode 8 (2002) include an entire section (Chapter 7) dealing with seismic design of composite structures; similarly, in Italy, the new

standard Ordinanza 3274 (2003) was issued. Two design approaches are implemented in Eurocode 4 (1994): the first is general and provides merely basic principles, the second is more specific although applicable in limited cases. The latter is reviewed hereafter. The assumptions of the refined method include: sections with double symmetry, given values of slenderness rations for bare steel sections, the mechanical ratio of the steel reinforcement should have lower and upper bounds, the relative slenderness λ of the column, as defined later, should be lower than 2. For fully encased columns, minimum covers should be ensured to protect bare steel from corrosion and prevent local buckling; minimum longitudinal and transverse reinforcement is also required. The code formulae are calibrated on the specific grades of the construction materials used in composite systems. In particular, ordinary grades of concrete, i.e. those with compressive strengths less than 50 MPa, can be used. For steel components, nominal yield strength greater than 355 MPa (Fe510). The design method is based on the partial safety factors. Allowable stress design is inadequate to account for all inelastic sources, especially for the steel components. The section analysis is based on the constant stress distribution, i.e. steel and concrete are characterised by rigid-plastic behaviour. As a result, high values of ductility are required; these can be achieved through the limitation of local buckling and adequate confinement. It is sufficient to satisfy merely the equations of equilibrium. For example, the design plastic resistance Npl.Rd is as follows: Npl.Rd = Aafyk/γa + Ac(0.85 fck/γc) + Asfsk/γs

(1)

where A, f and γ are respectively the area, the strength and partial safety factors of the materials. The sub-scripts a, c and s refer to the structural steel, concrete and reinforcement steel. The coefficient 0.85, used for the concrete strength, is not applied in the case of concrete filled sections, to account for the beneficial effects of the confinement. To perform the checks for composite columns under compression, a reduction factor χ is employed to erode the plastic resistance of the member. The latter depends on the member slenderness λ , which is generally given by: −

λ = N pl N cr

(2)

where Ncr is the elastic critic load of the column as follows: Ncr = π²(EI)e/ lo²

(3)

and Npl is the plastic resistance of the section, computed through eqn. (1) by setting all partial safety factors equal to unity. To estimate Ncr, it is needed to determine the length lo as well as the effective flexural stiffness (EI)e: (EI)e = EaIa + EcdIc + EsIs

(4)

where E and I correspond to the elastic modulus and inertia of the cross-section, respectively. To account for the nonlinear behaviour of concrete a reduced elastic modulus Ecd is employed. It follows that the check is given by:

NSd ≤ χN pl.Rd The checks for combined axial loads and flexure is carried out by means of interaction domains M-N. Such domains are reduced to account for the nonlinear behaviour of concrete and the member height. In particular, by using the non-dimensional domain N-M as per Figure 2a, μk is the bending moment corresponding to the axial load χ; χd is the design dimensionless axial

(5)

load which corresponds to μd. Assuming a linear distribution of bending moment along the column, it follows: χn = χ (1 - r)/4

with χn ≤ χd

(6)

where r is the ratio between the bending moments at both column ends. This ratio may vary between 1 (constant moment distribution) –1 (bi-triangular moment distribution); in turn, χn ranges between 0 (constant moment) and χ/2 (bi-triangular moment). Geometric nonlinearities are accommodated by amplifying the applied flexural moment through a coefficient given by the ratio between the design axial load Nsd and the critical load Ncr. As a result, the final check is performed by means of:

MSd ≤ 0.9μ ⋅ M pl.Rd

(7)

where 0.9 accounts for of the assumptions in the calculations, especially the depth of the stress block which is higher than that commonly used in the RC cross-section analysis. Early attempts to investigate the structural response of composite columns date back to the 70’s (Roik and Bergmann, 1989); they form the basis of the design rules implemented in Eurocode 4 (1994). Cosenza et al. (1998) reviewed recent experimental tests carried out worldwide: it is noted that the available data are applicable to member with moderate slenderness while the results are extrapolated for very slender members. Figure 2b compares the resistance derived from tests carried out by different researchers and the code design formula in EC4. It is evident that the code provisions lead to reliable estimates for all the cases fulfilling the assumptions in the design standards . N/ N pl.Rd dominio di interazione della sezione trasversale

1,0 χ

2.50

NTEST/(χNpl.Rd)

2.00

χd

μ

χn O O

μk

μd 1,0

M / M pl.Rd

Non-dimensional flexural resistance: μ = μd – μk (χd - χn) / (χ - χn) (8)

1.50

1.00

riempite circolari Filled circular riempite quadrate Filled square rivestite Encased Filled circular c.n.c.f.n. riempite circolari Filled square c.n.c. f.n. riempite quadrate Encasedf.n. c.n.c rivestite

0.50

0.00 0.00

c.n.c.= code not compliant f.n.= fuori norma

0.40

0.80

λ

1.20

1.60

Figure 2. – Axial load-bending moment domain (left) and comparisons between numerical and experimental tests (right).

3. Composite frames 3.1 Structural response and method of analysis Composite frames are generally more flexible than RC counterparts. In fact, as in bare steel constructions, the joint behaviour, particularly beam-to-column and base-column, is significantly influenced by its technology, i.e. welded and/or bolted. The alternative solutions give rise to lateral stiffness, strength and ductility which are very different from those exhibited by RC systems. Joints are thus of paramount importance in composite structures (Tschemmernegg, 1999) and the prediction of their response is generally not straightforward. Recent experimental and numerical investigations carried out by Fabbrocino et al. (2004 & 2005) were carried out jointly at University of Naples and Sannio, in Italy, to compare the seismic response of traditional and innovative base-column

2.00

connections as per Figure 3. It is found that traditional base-column joints behave poorly under cyclic loads, while innovative joints exhibit enhanced energy dissipation capacity. Composite column

Precast Composite column

Steel plate Foundation block

No-shrink filling grout Anchor bolts

Socket foundation block

Figure 3. – Layout of the sample base column connection: traditional (left) and socket-type (right).

Interaction phenomena between concrete and bare steel components may affect significantly performance, both under static and dynamic loads. Interaction between steel and concrete may augment the flexural rigidity of structural components; however, it may generate local over-stresses. The joint response may range between simple pin and fully rigid. In the former case the stiffness of the joint is much lower of the connected members; by converse, rigid joints are those with high stiffness and strength and hence can transfer the flexural forces of the adjacent beam-columns. Detailing of rigid joints is often cumbersome and/or expensive; semi-rigid joints are a viable alternative as they may possess adequate stiffness, strength and ductility. Typical layouts of composite joints can vary from rigid to simple or pinned (Leon and Zandonini, 1992). When pinned joints are employed, the slab is discontinuous to prevent the transferring of axial loads. Alternative joint layouts exist, they can incorporate top and base bolted cleats and shear tabs and can be also semi-rigid. On this subject extensive research activity on semi-rigid joints is also under development for seismic applications; a large number of tests have been carried out on beam to column sub-assemblages (Braconi et al., 2003; Salvatore et al., 2004) and modelling issues are currently analysed. Typical composite beams include steel beams and composite slabs, either solid or with sheeting. Simply supported beams are those with optimised structural performance, because concrete is in compression and steel in tension. Notwithstanding, continuous beams are often utilized to minimize the deflections and the joint zones. As a consequence, several experimental and numerical tests have been initiated to assess the structural performance of continuous beams under different load conditions. Connectors between steel and concrete components influence significantly the stiffness, strength and ductility of the composite systems. Such connectors can be designed with different degree of interaction. For partial interaction, the weakest links are the shear studs themselves; their response affects the global structural behaviour (Fabbrocino et al., 2000). Under earthquake loading, the response of composite systems is influenced by the interactions of all components, as shown by the surveys carried out in the aftermath of the 1994 Northridge (California) and 1995 Hyogoken-Nanbu (Japan) earthquakes (Matsuo et al., 2000; Yang and Tagawa, 2000). However, the response can be predicted if the ductility of components (beams, columns and nodes) is known (Bursi and Gramola, 2000; Bursi and Caldara, 2000; Plumier et al., 1998). In fact, within the framework of capacity design, the global behaviour relies on the available local ductility of structural components and their connections. The deformation capacity of multi-storey frames under earthquake loads can be assessed by means of inelastic static analyses (pushovers). These analyses provide the

progressive collapse of the structure under monotonically increasing lateral load patterns. The structural capacity is quantified by means of base shear (F) versus the total lateral drift of the roof. (Δ). The reliability of pushover curves is, however, limited to structures fulfilling regularity in plan and elevation. Pushover analyses provide a full description of the elastic and inelastic deformation capacity of the structures. Different limit states can be checked at each analysis step, both in terms of strength and deformation. It is thus an efficient tool that can be conveniently used in displacement-based assessment of composite structures. Nonlinear analyses are however time-consuming for applications in ordinary design offices. Alternatively, sub-assemblages can be assessed as per Figure 4; this approach is, however, applicable for regular systems (Bonacci and Wight, 1996). Axial and shear actions to account for are evaluated at the inflection points; these actions can be increased monotonically as in the pushovers and equilibrium equation used to perform the checks at each step. The relative displacement is obtained as: sag Δ = Δhog beam + Δ beam + Δ col + Δ jo int

(9)

where the first and second terms include the contribution of positive (jogging) and negative (sagging) bending moments. The effect of the column and joints are also to be accounted for (Fabbrocino et al., 2001). The contribution of the sagging moment increases as the rigidity of the joint increases. LL N N

VVcc

Η H

VFb t

Ftb V VVc c N

Figure 4. – Sub-assemblage used to compute the deformation capacity of composite components.

3.2 Codes of practice. The design of composite structures under static loads should conform to national recommendations, e.g. CNR 10016, and international standards, e.g. Eurocode 4. The rules implemented in these codes are generally characterised by high levels of reliability. However, for seismic design of composite structures, the reference codes are the OPCM 2003 and the Eurocode 8. The latter incorporate some rules, especially with regards to joint behaviour, which require further investigations. In so doing, several research projects were recently launched in Europe. Their primary aim is to assess the seismic design and response of composite framed structures which may dissipate earthquake-induced energy by means of flexural behaviour of beams and semi-rigid joints (Bursi et al., 2004; Braconi et al., 2003, Salvatore et al., 2004). To-date, the seismic design of structural systems can be as follows: a) Dissipative structures with composite dissipative zones; b) Dissipative structures with bare steel dissipative zones; c) Non dissipative structures. Composite components and/or joints can thus undergo inelastic deformation and dissipate energy. For dissipative structures with bare steel dissipative zones, the composite components should fulfil the rules in Eurocode 4 (1994) and concrete should not interact with steel in dissipative zones.

Non dissipative systems behave elastically and hence their performance can be assessed through elastic analyses account for member cracking. The structural design conforms to Eurocode 4 and rules on detailing (see for example Figure 5) are given to improve the available ductility and prevent local buckling and/or yielding occurrence. Multi-storey framed structures may dissipate large amount of seismic energy through global mechanisms. The latter can be achieved by forcing flexural hinges at both beam ends and at the column bases. Soft storeys and other local mechanisms should be prevented as they are associated with limited energy dissipation capacity. The sensitivity to second-order effects can be prevented by checking the following limitations: θ=

Ptot .d r ≤ 0,10 Vtot .h

(10)

Earthquake-resistant systems with reduced lateral deformation are defined “braced” or “non sway”; the second-order effects are not significant. Generally, if the design is primarily based on the requirements at serviceability limit state, the frame is not sensitive to second-order (or P–Δ) effects. The use of semi-rigid connections may undermine the satisfaction of the check in eqn.(10); in this case braced bay can be adopted.

h=hc

h=hc

b=bc

b=bc

tf

tf

tw

tw

Figure 5. - Stirrups welded to the web (left) and to the flange (right) of beam-columns.

4. Conclusions Composite steel and concrete systems are a viable alternative to both bare steel and reinforced concrete structures. They exhibit enhanced stiffness, strength and ductility. Moreover, their technology allows an easy of construction along with economy. Composite columns are structural members which benefit more of the composite action. In fact, concrete cover and/or filler prevents the occurrence of local buckling; in turn, steel hollow section enhances the concrete confinement. Additionally, fire and corrosion resistance can be achieved by using ordinary thicknesses of concrete. Composite frames benefit of the improved performance of steel and concrete columns; beams are generally in bare steel to yield at an early stage in compliance with the capacity design rules. Recently, different codes of practice have been issues for both static and seismic loads. However, the implemented provisions should be further investigated and their reliability re-assessed. Interaction between steel and concrete, beam-to-column and base column connections require additional extensive experimental and numerical work as the corresponding design rules relies on limited datasets. References Aval, S.B.B., Saadeshvaziri, M.A. and Golafshani, A.A. (2002). Comprehensive Composite Inelastic Fiber Element for Cyclic Analysis of Concrete-Filled Steel Tube Columns. Journal of Engineering Mechanics, ASCE, 128(4), 428-437. Basu A.K. and Somerville W. (1969). Derivation of Formulae for the Design of Rectangular Composite Columns, Proceeding of Civil Engineering Department, London, Supplementary Volume, 233-280.

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