Connections are one of the structural elements which is used for connecting various elements of a steel framework. An accumulation of different elements such as beams, columns which are connected to one another, usually at the end of the member closures such that it is made as a single composite entity. Connections are distinguished based on the medium used for connection. It is according to the type of internal forces, structural elements and type of members joining.
Technically, Simple connections are restrained connections that are supposed to transfer end shear alone and to have small confrontation to rotation. Thus, major moments at ultimate limit state should not be transmitted. This defines that the design of braced frames with multiple storeys are designed as ‘simple construction’, in which the beams are to be considered as simply supported while designing and the vertical members are designed for axial load and the small moments induced by the end reactions from the horizontal members. The steel frame is provided with the bracings or with the concrete core for more stability. Two standard types of simple connection are:
• Flexible end-plates and
• Fin plates.
Generally used simple connections in steel construction consist of:
1. Beam-to-beam and beam-to-column connections
2. Partial depth end plates
3. Full depth end plates
4. Fin plates
5. Column splices (bolted cover plates or end plates)
6. Column bases
7. Bracing connections (Gusset plates)
Simple connections are necessary for skewed joints, beams which are eccentric to columns and column web connections. These are categorized as special type of connections and separate care is taken from the conventional procedures.
Simple connections are comparatively economical to manufacture than moment-resisting connections, since it needs very less effort for fabrication and especially in welding process.
Guiding on costs is difficult as a Steelwork Contractor, labor rates and their wages can vary considerably and are reliant upon the level of investment in plant and equipment. However, reducing the work load is the main aim of a steel contractor. The cost of materials for the purpose of fixing bolts is very much lesser than the expenses on labor to perform welding tasks. In a steel workshop, the fabrication cost of steel connection portion is 30 – 50% of the total cost incurred for fabrication.
Standardized connections are proficient in their production. Steelwork Contractors arm their workshops with special equipment that increases the speed of fabrication. This allows manufacturing the fittings and members more rapidly than it could, if the configurations were non-similar.
The standardized details mean the steelwork is all set to upright, which ensures a safe working constructional area for the steel erectors.
At the end of the service life of a structure, it is evident that the connections are detachable at the joints but it can be made possible only on bolted joint. One of the major advantages is, the steelwork can either be reused or recycled after dismantling. Hence, it helps in reducing the environmental consequences of its construction.
1.2.1 Beam-Beam and Beam-Column Connections
The following design procedure can be suitable for both manual calculation and computer software. The strength verification of a pinned joint undergoes three stages:
• The joint detailed is ensured as it develops only nominal moments which do not adversely affect the members or the joint itself. It behaves as a ductile portion such that its detailing should be given.
• The load path throughout the joint should be identified.
• The resistance of each component should be checked.
When the components of beam-beam or beam-column connections have to be designed, ten design checks should be done for vertical shear.
For the verification of the joint resistance, six more checks are necessary other than these ten checks. Only beam-column joints should restrain the lateral tying forces if all the other means cannot able to restrain the forces in a structure.
1.2.2 Flexible End Plate Connections
In the workshop, the supported beam is welded along with the end plate, which could be either partial depth or full depth. Then, the beam is bolted to the supporting beam or column; this can be done on the site.
This flexible end plate type of connection is inexpensive compared to all the other connections. But the only disadvantage is that there is less chances of site corrections.
These end plates are the highly accepted beam connections by the steel contractors. It can be used along with other small beam called skewed beams.
To connect hollow section columns, special assemblies such as Flow drill, Hollow-Bolts, Blind bolts are used. Requirements of steel rod detailing and design checks for partial and full depth end plates joint, which are appropriate to beam-beam connections and beam-column connections as well.
1.2.3 Fin Plate Connections
These connections are inexpensive to manufacture and effortless to upright. Fin plate connections are popular in the steel construction industry, as they can be the fastest connections to upright and it also helps in overcoming the difficulty of shared bolts in two-sided connections.
A fin plate connection consists of a long length of a plate throughout. This length of plate welded to the supporting member in the manufacturing site (which may be a beam or a column). This welded member is connected to the supported beam web which is bolted on the construction site. There is a little space between the supported beam end portion and the supporting column.
In fin plate connection design, it is essential for the shear to recognize the suitable line of action. There are two potential: either the shear acts at the column face or it acts along the bolt centre group which enables connecting the fin plate to the beam web.
Fig 1.1 Types of Fin Plate Connection
1.2.4 Column Splice Connection
In multiple story construction, Column splices are compulsory to afford strength and stability to the both axes of the column and its rigidity. The rolled I and hollow section elements are bolted as column splices are shown in the fig 1.2.
Splices are located usually over the floor level just about 600mm and are provided at every two or three stories of a building. This effects in expedient lengths for manufacturing, transportation and erection, and allows easy contact for bolting up from the nearby floor on construction site. The stipulation of splices at each storey level is rarely economical since the column material costs saving are normally beyond the material, manufacturing and erection costs of the column splice provision.
These type of connection is again classified in to two main subcategories:
• Bearing type,
• Non- Bearing type.
In bearing type column splices (fig.1.2), the upper shaft transfers the loads in direct bearing directly or via a division plate. The simplest connection of all connections is the bearing-type column splice. Very few bolts are needed in bearing type splice than the latter. It is the reason for the high usage of bearing type column splice in steel construction.
A conventional connection can be put in to use, when no net tension is present. On the other hand, the importance of splices and bolts in transmitting the maximum compressive force by at least 25% is explained with code stipulations. Tying resistance of the splice connection check is considered as a critical one for bearing column splices.
Splices classified as non-bearing type transmits loads through the bolts and splice plates. Direct bearing between the members should be neglected, the connection being detailed with a corporeal gap involving the two shafts. The design of a non-bearing splice is more concerned, as all forces and moments must be transferred via the bolts and splice plates.
The moment due to strut action is considered not very important, as the splices are accommodated above the floor level. Though, the moments induced in splices placed other than those positions should be taken into report.
Column splices should contain the connected members in line. The members should be prearranged so that the centroidal axis of the splice material matches with the centroidal axis of the column sections over and under the splice. If the column sections are counterbalance the moment due to the eccentricity should be taken in to account in the joint design.
Fig 1.2 Types of Column Splice
1.2.5 Bracing Connections
Flats, angles, channels, I sections, and hollow sections comes under Bracing members category. Bracing arrangements may alter the bracing members working in tension only or in tension and compression together. In the majority of the cases, the bracing member is joined by bolting to a gusset plate, which is welded to the beam/ column or usually welded to the beam and its end connection. The details of bracing connection are shown in the figure 1.3.
Assuming all forces intersect on member centrelines, bracing systems are analysed. But, realising this assumption in the connection details may end up in a a very large gusset plate connection, particularly if the bracing is shallow and sharp
Bracing connections are usually made with bolts which are non-preloaded in clearance holes. This allows less resistance from movement in the connection, but in practice this is disregarded in traditional construction. The general design process is:
• The load path is to be identified through the connection
• The connection should be arranged in such a way to ensure that the design objective of the members is realized.
• The effects of any significant eccentricity to be included
• Connection components should be checked.
Fig 1.3 Bracing Connections
1.2.6 Riveted Connections
A rivet is a stable mechanical tie. Before being installed, a rivet consists of a soft cylindrical duct with a head on one end. The other end to the head is called the tail. On fixing the rivet is placed in a drilled hole and the tail is upset or deformed, as it increases to 1.5 times the actual shaft diameter, keeping the rivet in place. To differentiate between the two ends of the rivet, the head is called as the factory head and the tail end is called as the shop head.
Because there is effectively a head on each end of an installed rivet, it can support the loads parallel to the axis of the shaft. On the other hand, it is able to support shear loads which are the loads perpendicular to the axis of the shaft. Bolts and screws are better appropriate option for tension applications. Conventional riveted connections are made up of round ductile steel bar called shank. The length of the rivet should be adequate to form the second head. Design of this connection is same as the bearing type of bolted connection.
During World War II, Rivets were used in many tanks by a many countries which include the M3 Lee produced in the United States. yet, many countries learned that rivets were a major disadvantage in tank design, because if a tank was hit by a large shell it would displace the rivets and they would flutter around the inside of the tank and hurt or kill the crew, even if the projectile didn’t break through the body armor. Some countries such as Italy, Japan, and Britain used rivets in tank designs all the way throughout the war for different reasons. Blind rivets are used almost across the world in the fabrication process of plywood roads.
Fig 1.4 Riveted Column Splice
1.2.7 Bolted Connections
In bolted connections, beam and column are fixed firmly together mainly by bolts. Bolts could be loaded in tension or shear and sometimes both tension and shear. If threads of bolts under shear force expelled, it highly increases strength. If threads of bolts under shear force included, it results in decreasing strength. Bolts are of two types: bearing type bolts and High Strength Friction Grip bolts (HSFG).
Bearing type bolts are again differentiated in to black bolts, turned bolts and ribbed bolts. Black bolts are ordinary, rough bolts which are very cheap. Above all, black bolts can be used in light structures such as trusses, purlins etc., under static load. An example image of bolted column splice at the joint is shown in the figure 1.5.
Fig 1.5 Bolted Column Splice
1.2.8 Welded Connections
In welded connections, structural members are fixed together mainly by welds. Welds are classified are groove, fillet, plug, slot and plug and slot weld. Groove welds are more consistent than others. Most commonly used type of weld is fillet weld. This fillet welds are weaker than groove. Plug welds and slot welds are pricey and it involves bad transmission of tensile forces. Plug and slot welds connects diverse parts of members together.
Welds can be located in horizontal, vertical, overhead and flat. Welded connections are inexpensive in purchasing materials and labor wages. It is most reliable and efficient type compared to rivets. Fabrications of complex structures are effortless like circular steel pipes. It also offers rigid joints. There is very less provision for expansion or contraction. As a result of it, there are greater chances of cracking. Due to irregular heating and cooling, member may deform and it may ends up with additional stresses.
Inspection is complex and more expensive than rivets. Bolted-Welded connections are welded in shops and can be bolted on site. It is cost effective since it provides better strength and ductility comparing with other types. Shear connections lets the beam end to rotate without a major restraint. It helps to transmit shear from the beam to other members. Commonly used shear connection types are double clip, shear end plate and fin plate.
Moment and shear are resisted by designing the moment connections. Moment connections are usually referred to rigid or fully restrained connections. The connected members are provided with full continuity. It is designed to bear the full factored moments. The main cause is that the building has to resist the consequence of lateral forces such as wind and earthquake.
Fig 1.6 Welded Column Splice
1.3. FINITE ELEMENT ANALYSIS
Finite Element Analysis (FEA) is a automated method of forecasting how a product responses to forces, vibration, heat, fluid flow, and other physical characteristics. Finite element analysis gives you an idea about whether a product will fracture, rupture, or work the way it was designed. This definition is what we call as analysis. However, it is used to foreseen what is going to happen when the product is put in to use. This works by splitting down a real object into a large number of tiny elements such as small cubes. Mathematical equations help to calculate the performance or response of each element. A computer then sums up all the single behaviours to determine the behavior of the actual object.
Finite element analysis helps in determining the behavior of products that are influenced by many physical effects which includes:
• Mechanical stress
• Mechanical vibration
• Fatigue
• Motion
• Heat transfer
• Fluid flow
• Electrostatics
• Plastic injection moulding
Some modern FEM packages include definite components such as thermal, electromagnetic, fluid, and structural working environments. In a structural simulation, FEM helps extremely in creating stiffness and strength visualizations and also lessens the weight, materials, and costs.
FEM envisions thorough visualization of where structure bend or twist, and shows the distribution of stresses and displacements. FEM software gives a wide variety of simulation options for manipulating the difficulty of both modelling and analysis of a system. Similarly, the preferred level of accuracy required and associated processing time requirements can be managed at the same time. FEM let’s to construct, refine and optimize the entire design before it is manufactured.
This influential design tool has considerably enhanced both the standard of engineering designs and the methodology of the design process in many work applications. The introduction of FEM has significantly reduced the time to take products from concept to the production line. It is mainly through improved initial sample designs using FEM that testing and development have been accelerated.
As a outline, advantage of FEM include improved accuracy, better design and enhanced insight into critical design parameters, virtual prototyping, fewer hardware prototypes, a quick and inexpensive design cycle, increased productivity with a increase in revenue.
FEA has also been planned to use in stochastic modelling for numerically calculating probability models.
1.4 ABAQUS SOFTWARE
ABAQUS-FEA Software which is formerly called as ABAQUS. It is a software package for finite element analysis and computer-aided engineering, originally released in market in the year of 1978. This software’s name and logo are from the idea of abacus calculation tool.
Abaqus is able to pre-processing, post-processing, and monitoring the optimizing stage of the solver. However, the initial stage can also be done by other compatible CAD software or even with a help of text editor. Abaqus/Standard, Abaqus/Explicit or Abaqus/CFD has the potential of achieving the processing stage. Dassault Systems also produces Abaqus for CATIA for advanced processing and post processing stages to a pre-processor like CATIA.
The Abaqus product suite consists of four core software products:
Abaqus/CAE is an acronym of “Complete Abaqus Environment” with a clear root in Computer-Aided Engineering. It is a software application which is used for both the modelling and analysis of mechanical components and parts. It helps in visualizing the finite element analysis result as a final stage of this software. A division of Abaqus/CAE together with the post-processing unit can be launched separately in the Abaqus/Viewer product.
1. Abaqus/Standard: it is an all usual purpose Finite-Element analyzer which employs implicit integration scheme.
2. Abaqus/Explicit: this type is a special-purpose Finite-Element analyzer which uses explicit integration scheme to resolve highly nonlinear systems with many composite contacts under transient loads.
3. Abaqus/CFD: it is another type from Abaqus package which is a Computational Fluid Dynamics software application that provides highly developed computational fluid dynamics capabilities with widespread support for pre-processing and post processing provided in Abaqus/CAE.
4. Abaqus/Electromagnetic: it is a rarely used type in this software which is a Computational electromagnetic software application, solves advanced computational electromagnetic problems.
Entire finite-element analysis consists of 3 separate steps:
• Pre-processing or modelling: This stage involves creating an input file which contains an engineer’s design for a finite-element analyzer which is also referred as “solver”.
• Processing or finite element analysis: This stage produces an output diagrammatic visual file.
• Post-processing: In this stage, creating report, image, animation, etc. from the output file is done. This stage is called as a visual rendering stage.
1.4.1 Applications
Abaqus is primarily used in the field of aerospace, automotive and industrial products. The product is accepted and famous with academic and research institutions because of its extensive material modelling potential and the software’s ability to be modified. Abaqus also provides a good compilation of Multiphysics characteristics such as coupled acoustic-structural, piezoelectric, and structural-pore capabilities making it smart for production-level simulations where various fields need to be coupled. Abaqus was originally designed to deal with non-linear physical behavior. As a result, the software has an wide range of material models such as rubber-like material capabilities. Other FEA Softwares used for analysis are listed below:
1. Advanced Simulation Library
2. ANSYS
3. CLAWPACK
4. Code Saturne (GPL)
5. Coolfluid (LGPLv3)
6. COMSOL Multiphysics
7. deal.II
8. FEATool Multiphysics
9. FreeCFD
10. Gerris Flow Solver
11. Nektar++
12. OpenFVM
13. SU2 code (LGPL)