Hybrid Structures: Steel and Concrete

• Concrete and steel work well together due to similar thermal expansion, which ensures deformation compatibility and minimizes internal stresses with temperature changes.
• Steel is protected from corrosion when embedded in concrete's alkaline environment.
• Concrete enhances fire resistance, while steel bonds well with concrete.

Challenges with Traditional Reinforced Concrete

• The traditional use of reinforced concrete structures, as developed by Joseph Monier in the 19th century, does not always satisfy modern client and architectural needs.
• Reinforced concrete's substantial self-weight and limited tension capacity are major drawbacks.
• Prestressed concrete was invented in the early 20th century to reduce said drawbacks.

Hybrid Structures Defined

• Hybrid structures combine steel and concrete and differ from classic reinforced concrete in Eurocode 2 and composite structures in Eurocode 4.
• Hybrid structures are reinforced concrete structures that incorporate steel, composite, or concrete elements, reinforced with embedded steel parts.
• These structures do not need a continuous steel skeleton.
• The continuous steel skeleton design is the only design type referenced in Eurocode 4.
• Hybrid structures are increasingly used to reduce structural component sizes and improve execution quality by minimizing on-site congestion.

SmartCoCo Project

• Significant knowledge gaps existed regarding hybrid structure components, leading to unanswered design questions.
• Addressing these gaps fostered collaboration between designers and university researchers.
• The University of Liege initiated a research project in 2012 to investigate hybrid structures.
• The SmartCoCo project collaborated with Imperial College (UK), INSA Rennes (France), ARCELOR MITTAL (Luxemburg), and BESIX (Belgium).
• The Research Fund for Coal and Steel of the European Union (RFCS) provided funding.
• Research examined various hybrid situations, producing a series of reports.
• Project reports include the following:
  • An up-to-date report with existing information, presented in Plumier (2016).
  • A generic design approach report with tentative design suggestions for test specimens.
  • Specimens design and test reports.
  • A calibration report assessing tentative design method validity using test results.
  • A design guide providing practical design indications based on research.
  • An executive summary report (SMARTCOCO, 2017) available on the internet, with detailed reports available upon request.

Book Overview

• This book builds upon the SmartCoCo project, enhancing clarity in presentation.
• It offers practical design methods for concrete structures reinforced unconventionally with steel profiles.

Marine-Piled Concrete Structures

• Prefabricated components are essential in marine-piled concrete structures due to the inability to use scaffolding above water.
• Prefab planks are temporarily placed on steel piles.
• Temporary connections consist of a steel profile protruding from precast planks.
• The steel profile must withstand the self-weight of the precast plank, the added steel cage, and the fresh concrete's weight to form structures like monolithic mooring dolphins or marine platforms.
• Force transmission from steel to reinforced concrete is challenging.
• Steel profiles often use welded built-up sections to reduce precast plank thickness and self-weight.
• Force transfers from the steel profile to the concrete using two statically valid mechanisms, with each mechanism transferring a portion of the action effect F proportionally to its relative stiffness.

Chapter Summaries

• Chapter 2 focuses on force transmission between concrete and embedded profiles.
  • Challenges include combining resistance from bond, friction, connectors (studs/plate bearings), and reinforcing concrete in transition zones to prevent local damage.
• Composite components are essential in high-rise buildings.
  • Composite components minimize structural component size and ensure vertical/lateral stability.
  • Design begins with classic reinforced concrete for economic reasons.
  • Maximum dimensions of vertical load-bearing components are often predefined due to architectural or client needs, necessitating high concrete grades or additional embedded steel profiles.
• Chapter 3 gives design recommendations for hybrid structures using linear and non-linear analysis.
  • Presents a simplified method to analyze slender hybrid components with axial force and uniaxial bending moment, using dedicated software from the SmartCoCo project.
• Chapter 4 explains calculating action effects and resistance of walls/columns with embedded steel profiles.
  • The chapter includes reinforcement details necessary for component design.
  • The chapter also discusses advantages like ductility, reduced reinforcement congestion, shear stiffness, and resistance to in-plane effects.
• Figure 1.3 presents a real-world example of a concrete column with embedded steel profiles in the lower levels of a 54-floor residential tower.
  • Local strengthening addressed high normal force and column slenderness from increased story height in the podium/lower stories.
  • The wider bottom or podium of a high-rise building diffuses vertical force via diagonal compression struts from the podium-tower transition, as shown in Figure 1.4.
  • The stress on inclined struts depends on foundation stiffness and soil-structure interaction.
• Potential differential settlement between the central and podium parts of the foundation can increase force diffusion and local stress in diagonal compression struts.
  • Classical reinforced concrete may not be suitable near openings next to inclined struts, leading to local hybrid solutions.
• Figure 1.4 illustrates lintels connecting main core walls with external wing walls.
  • These linking elements endure high shear and bending moments, especially where MEP (mechanical, electrical, and plumbing) openings are present.
  • Specific embedded steel profiles (Figure 1.6) ensure lateral building behavior.
  • Reinforced concrete lintels could cause significant cracking, leading to an uncoupled-wall structural system and limited shear transfer via the lintels, reducing building resistance and causing unfavorable stress distribution at the foundation level.
• Strong coupling beams are necessary for high-rise towers.
  • In the Mohammed VI tower in Rabat, lateral deformation under wind was the primary design criterion.
  • A tuned mass damper (TMD) was used at the top for inhabitant comfort.

Lateral Stiffness Requirements

• Inter-story drift should be limited to hs/300 at each level to ensure compatibility between the main structure's expected deformation and acceptable façade deformation.
• Limit general lateral deflection at the top to H/450 for a 100-year return period wind speed, which will limit higher bending moments of second-order effects at the tower's bottom.
• Lateral stiffness depends on the core walls working together with strong coupling beams.
• The plan view (Figure 1.8) indicates the location of the coupling beams ("Linteaux Nord" and "Linteaux Sud"), where false ceilings limit space for MEP ducts, requiring large holes to exist in the coupling components.
• Classical concrete beams are insufficient due to potential cracking.
• Figure 1.9 illustrates that steel coupling beams ("Vierendeel" beams) maximize MEP duct space.
• Action effects within each profile combine moment, shear, and axial force, as detailed in Figure 1.10.

Steel Structure Connections

• Chapters 5 and 6 offer design guidance for steel structures to optimize local performance.
• The transition zone between steel profiles and reinforced concrete requires proper design for transmitting axial forces, shear, and bending moment carried by the interrupted steel element, as illustrated in Figure 1.11.
• The design of Abu Dhabi’s National Oil Company (ADNOC) headquarters (342 meters, built 2010-2016) presented unique challenges.
• Architectural needs limited vertical bearing element sizes on a typical floor (Figure 1.12).
• Concrete-filled tubes (CFT) were chosen for execution and quality, but specific beam-to-column moment connection designs faced difficulties due to limited scientific knowledge and design recommendations.
• Chapter 6 of this book explores steel shear keys as connectors between reinforced concrete beams/flat slabs and steel columns.
• Figure 1.13 shows the solution implemented in the 342-meter tower.

Chapter 2: Load Transfer at Steel-Concrete Interface

• Longitudinal shear force transfer between concrete and steel components involves:
  • Chemical bond: cement paste adhesion to steel.
  • Mechanical bond: Resistance to relative movement due to interface roughness.
  • Friction: Proportional to normal force at the interface.
  • Mechanical interaction: Ribs, shear studs, or plate connectors.
• Chemical bonding is usually ignored, but friction and mechanical actions are critical. Mechanical connectors provide composite action in flexure between concrete slabs and steel sections.
• They distribute inertial forces to lateral load-resisting elements or enhance connections in composite columns.
• Headed shear connectors are welded to steel beams for varying connection degrees.
• There are many shear connector options for steel-concrete interfaces: embossments, ribs, plates, and studs.
• The type of shear connection impacts composite component behavior.
• Rigid connectors (plates) ensure full composite action, while flexible connectors allow partial composite action.
• Interlayer slip is crucial in the analysis procedure.
• Shear force transfer in composite components is measured experimentally, with the push-out test being the simplest and most common. Shear resistance versus behavior can be ductile or brittle.

Bond Stress

• Formulas often assume constant bond stress distribution at the steel-concrete connection surface for composite structures without mechanical connectors
• Roeder (1984) used push-out tests to study bond transfer in embedded composite columns (no shear connectors), concluding that bond stress is primarily from flanges and depends on concrete strength.
• Wium and Lebet (1994) found that average bond stress increases with load application until chemical debonding, then reduces and stabilizes to frictional resistance.
  • Thus, the authors suggest using only friction-based bond stress in composite structure design.
  • The bond strength (τmax) relies on concrete cover thickness, hoop reinforcement quantity, steel section size (depth), and concrete shrinkage.
• Wium and Lebet (1994) distinguished between first and second loadings
  • Experimental tests showed that when embedded coumn are loaded beyond chemical debonding and then unloaded and reloaded, the value of the maximum stress τmax did not change, but the force-transfer mechanism and the stiffness did changes.

Bond Stress Capacity

• Bond stress capacity is assessed via maximum load transfer divided by the embedded steel section's surface area.

Push-Out Tests

• Wium and Lebet (1991, 1992) studied steel reinforced concrete (SRC) composite column bond stress capacity using push-out tests
• Bryson and Mathey (1962) studied the effect of the surface condition of the the steel on the bond stresss capacity.
  • The steel was cleaned, sandblasted etc
  • sections that were sandblasted tended to develop large maximum average bond stress than those that had mill scale.

Casting Positions

• Hawkins (1973) examined the influence of position during casting and the relative size of reinforcement.
  • Specimens that were cast horiztonally had a smaller bond capacity compared to those cast vetrically.

Confining Reinforcement

• Amount of confining reinforcement did not consistently affect the bond stress capacity before significant lip.
  • After slip confining reinforcement increased the bond resistance.
• Roeder (1984) looked at distribution of bond stress over the component and specimen length.

Scope of Chapter 2

• Although encased SRC construction is frequently implemented there is not a lot of guidance in the design in the United States.
• American Concrete Institute (ACI) has reognized the use of SRC columns since 1995
  • Shear transfer beetwen the steel and concrete requires direct beraring
  • No allowance is made for a natural bond between steel and concrete.
• Japanese provisions permit the use of bond stress of 0.2f’c but not more than 0.45 MPa over the length of the component
  • This chapter discuses various guidelines provided by Eurocode 4 for load introduction and force-transfer mechanisms and combined with certain reccomendations based on recent experiemental results.

General Approach

• procedure for design concrete structed reinforced by steel profiles is neither availble in Eurocode 2 (2004) nor Eurocode 4 (2004).
• They offere guidelines to load and force transfer
• Irrespective of the structure type, transerve cracking and spilitting can occur in the conrete around the steel profile.

Experimental Enidence from the SMARTCOCO Research Project

• Ten tests were carried out
• Considering six different configurations
  • Flexible and stiff connecters
  • Differect confinement schemes and orientations of the steel profile
• The orientation of the steel profile affect the smallest dimension of the global composite specimem as listed below.
  • Steel profile with strong axis perpendicular to the longer wall face
  • Steel proflies withh a strong axis parrallel to the longer wall face
  • steel profile weak axis perpendicular to the longer wall face
  • steel profile strogn axis perpendicular to the longer wall face respecitvely with rust and paint on profile.
• A direct push out testswere performed on all the speicmens (FIgure 2.3 schematic diagram)
  • Connecters and vertical and horizontal reinforcements were all desinged basd on guidelines avaialbe in euro code 2 and 4
• Euro code 4 desgin rules offer a safe etsmation for the specimens
  • i.e. configurations A and C, independently of the orientation of the steel profile (web-oriented parallel or perpendicular to.

General Provisions For Composite Sections

• The availble provisions that are used to etsmaet the resitanceof composite croiss ections assume that no signfiicant slip occurs at the interface.
  • Provides ciratin principles, e.g., force and moments appleid between compsents (axial load)
  • provide a celarly defined laod that not involve a signfiicant amount of slip between the setel - concrte interface.

Load introduction in Hybird Components

• Primary concern that arrises for achieving a successful transfer of shear forces is to successfull transfer it betwen maerials
  • Headed Shear Studs, Perfobond Ribs etc
• Headed shear suts hae been the most pupular among all these options

Share Stud Connectors

• Load Transfers are through stud connectors.
  • Lungershausen (1988) Defined four component that contrbitue to load tranfer (Figure 2.5):
    • Initiallly, the maority of longitduintal shear forec is transfered at the base of stud (A)
    • The muti-axial high-bearing stresses eventually laed to local crushing filure of teh conrete at the botttom (B)
    • If the base is free too move laterally and the top cnat defome
    • To balance tehs tensile stresses, the compressice forces develop in the conrete which allows for additiaon frcictional froces.
    • Ventuylly the shear connection fails ahdn the shank experience shear and tension failure above hte weld collar.
  • Ultimate tensiel stregnth obviosuly influenct he shear stregnth ofthe stud directlty.

Longitudinal Shear Flow

• Concret beams istransfered across the setel - flage via the methaniacal acction
• Ability to trsnaprt longitdual shear froces depends on stregnth of the doel
• The stud is sunjected to shear and flexturl stress and the conrete zone is in compresseive stresses.