Category Archives: Tekla Structures

Joints in steel structures

The steel frame structures with connecting parts as bolts and welds are designed on the basis of forces resulting from plane frame analyses. The spatial behaviour of the structure due to the loading is most times neglected because the resistance of the traditional joints against these resulting forces is normally sufficient. The wind loading on a building can produce tensile forces in the joints, while eccentric loading on the floor can produce torsional moments on the joints. Traditional joints have by themselves sufficient resistance against these tensile forces and torsional moments. Because bolts in holes, designed primarily for shear and or bending moments, are almost always capable of carrying these “secondary” forces and moments. In fact, these forces are not “secondary” but primary forces and moments due to the spatial behaviour of the structures under the loading actions and most times neglected in design. In designing new concepts for plug and play joints the designer needs to be explicitly aware of these forces and moments, which can implicitly be neglected in designing traditional joints. Also, loading that can appear in the onsite construction phase produces most times tension and torsion in the beams and joints. The main task of the designer is to design the joints such that robustness, in order to prevent premature and progressive collapse, can be demonstrated explicitly. The distribution of forces and moments in the structure due to the loading is due to the strength and stiffness distribution in the structure. Thus it can be said that the stiffness, strength and rotation capacity, together with those of the structural components like beams and columns, produce these forces in the joints. The designing of the joints include the connecting parts are of direct influence on the level of forces and moments in these joints. Joining components such as columns and beams together is construction, while designing is making choices for components taking the structural properties such as strength and stiffness into account. In traditional design the joints are stiff and strong and that the forces and moments in the structure are determined using the linear-elastic theory. In many cases in practice, this is neglected. The joints strength is adjusted to the level needed. As a result, most joints have low deformation capacity. Last but not least, fabrication costs are very high. In modern design, the joints are columns and beams with properties as stiffness, strength and deformation capacity. These structural properties of the joints are incorporated into the design on the same level as those of columns and beams. The joint layout is influenced by the fabrication considerations and considerations for easy and safe construction on-site. The structural safety of all components depends on the design method used to determine the distribution of forces and moment in the structure.

Behavior of steel connections

Extended end-plate connections are widely used in steel structures as moment resistance connections and as an alternative to fully welded ones that have been considered for use in steel frames. These connections consist of end-plates welded to the end of beams and field bolted to the connecting columns. The understanding of the behaviour of joints is enhanced by developing analytical models. Various forms of analysis and mathematical modelling methods have been suggested to study the semi-rigid characteristics of beam-to-column joints and their influence on the response of the rest of the structural members. There is a simplified analytical model to represent the semi-rigid connection behaviour based on the moment-rotation characteristics of the joint. The component method does not apply to end-plate joints with pre-tensioned bolts and research into these types of end-plate connections are therefore much needed. Many types of research on end-plate steel connections have been carried out analytically and experimentally to determine an accurate method for predicting the connection rotational behaviour. Two experimental tests were conducted on beam-to-beam and beam-to-column joints. Both tests failed the column flange in the compression zone. There were also tests on three specimens to enable the prediction of the moment resistance and the rotation capacity about the minor axis of the beam-to-column semi-rigid joints. Thus it motivated the development of a mechanical model to assess the structural response of joints. Furthermore, connection types and details are numerous and innovative with many parameters, that must be accounted collectively to characterize the behaviour of the joint. Such parameters include whether the end plate is flush or extended, whether the end plate extends beyond one or both of the beam flanges, the diameter and the grade of the bolts, the number of bolt rows, the vertical and horizontal spacing of the bolts, the end-plate thickness, the stiffening of steel and the coefficient of friction at the contact surface. Because of this, it is almost impossible to study the behaviour of these joints comprehensively except by physical tests. However, due to a limited number of tests, the numerical simulations are widely used currently. The more accurate finite element model for parametric analysis is particularly important. The interaction between the different components of the joint should be taken into consideration in the modelling. The contact between these components, viz. end plate-column flange, bolt-column flange and bolt-end plate were represented using the surface-to-surface contact interaction technique.

Behaviour of Steel

End-plate connections have become more popular in steel building constructions due to their economy, simplicity of fabrication, and good structural performance. The end-plate connections have been presented with three typologies: header, flush and extended end-plate connections. From which five observations can be made: -The most studied are the extended and double end-plate connections.-Most models use four bolts and no stiffeners.-3D FE is mostly used for bolts, although truss, beams and plane stress elements have been used. The nut is less commonly modelled, however, when it is modelled, a3D FE is used. The extended and the flush end-plate 6-bolts unstiffened connections are usually analyzed. There are several analysis conducted using ABAQUS/Standard software. Both materials and geometry non-linearity were considered. This element helps avoid shear locking phenomenon (comparing with element C3D8R), which will significantly affect the initial stiffness of the connection. Since the solid elements have no rotational degree of freedom, the number of elements through the thickness of each component plate plays a critical role. For the regions where the hexahedra formulation was not possible to be used the wedge approach was used with an element designated by C3D6 which is a 6-node linear triangular prism and constant pressure element. To reduce the number of contact planes and the complexity of the model, the bolt nut forms an integral component with the bolt shank rather than as an individual part. Different mesh sizes have been examined to determine a reasonable mesh that provides both reliable results with less computational time. The results show that, if the mesh is too coarse, a convergence problem will occur as the contact element was used between the column flange and the endplate surface. However, if the mesh is too fine, the computational time will be excessive. The finite element mesh adopted for all joint components with the smallest and largest element sizes being 5 mm and 50 mm, respectively. The fine mesh is thus created at the region around the bolts and studs to achieve reliable results. The finite element mesh of each specimen contains a lot of elements. The material behaviour used for the joint is represented by the bilinear plus nonlinear stress-strain curve and the plasticity behaviour in the connection was represented by the isotropic work hardening assumption.

Optimization of steel structures

Typically in the optimization of steel structures, the weight of the structure is chosen as the objective. When comparing different steel grades, the total manufacturing cost is a more relevant criterion than weight, as has been pointed in other applications, as well. The material costs are typically 40 % of the total costs of steel structures. The manufacturing costs depend roughly on the steel material costs and actions which are needed in manufacturing. The material costs of HSS have been given in up to the steel grade S1000 based on interviews of five European steel fabricators. In this research, the same relational material costs were accepted for HSS by taking as a reference to the regular steel S355. In the action time, based calculation method to calculate the manufacturing costs of structures is proposed. A similar method has been used in. In all of these, the manufacturing costs were calculated based on a generic feature-based cost calculation method including costs of all actions in the workshop. In the developed method has been compared with a non-feature-based method of using data of an actual project. The authors are not aware of a cost calculation method that covers the data for steel grade up to S960. In this study, the industrial partners of the research project estimated the cost factors for manufacturing concerning S355 such that S960 could be included in the comparison. Optimization of typical steel structures, such as beams, columns and trusses, has been the subject of numerous research papers. Optimal properties for I-shaped beams have been considered since up to recent studies including steel grades up to S700 and hybrids where the flanges are made of different steel grades than the webs. Hybrids have shown to be cost-effective solutions for beams. Up to up to 34% weight savings for welded I-beams (WI-beams) were obtained with large loads, whereas about 10 % of cost savings were found using HSS instead of S355. In beams, the SLS may become critical, but using pre-camber, as is possible for welded beams, the deflections can be reduced. Welded box beams (WQ-beams) with wide bottom flanges are frequently used solutions in slim floors, especially in Scandinavia. Optimum solutions for WQ-beams using HSS is still a rather open question. Welded box (WB) columns include the possibility to use hybrid solutions but the optimal solutions for these are not known.

Braced Frames

Braced Frame is a common system employed to resist the significant lateral loads where when tall structures are exceptionally subjected to brace the frames, bracing can occur within a single bay inside the internal bays or along the external bays or it can span the entire face of a structure on perimeter. The advantages of braced frames from a structural engineering standpoint are enormous. Braced frames carry the lateral forces in an axial manner with tension and compression, rather than through the bending of elements which is quite inefficient from flexibility point of view. The separation of the lateral system from the gravity system being concentrate at some points gives further advantages during the design phase. This allows the lateral system to be separate , therefore permits for repetition in-floor systems and column sections. With minimal frame action and mostly axial deformation, minimal moments in the columns and girders result from the applied lateral loads compared to a moment frame. This in turn leads to cheaper girder-column connections. Among all Bracing Systems, two of them are normally widely used by the designers, namely, the cross Concentric Bracings System and the Eccentric Bracing System. These are explained further in detail as follows: Concentrically Braced Frames Steel Concentrically Braced Frames (CBFs) are assumed and recommended to be strong, stiff and ductile. The quality of the seismic response is determined by the performance of the brace. For achieving a good performance from a CBF, the brace must behave as a structural fuse thus should fail prior to any other component of the frame. The frame remains stable and resist gravity loads and withstand aftershocks without collapse. Braced Frames of structural members consists of a theoretical point of view, maybe connected to each other by means of simple flexural hinges. The resistance to horizontal forces, the most common type of braced frame is the concentric cross brace. The restricted space may have an effect on the mechanical and plumbing distribution as well as any architectural soffit details. The structural engineer needs to be able to provide this type of information to the architect to avoid potentially costly field revisions during construction. With this design approach, only half of the members are active when the lateral loads are applied. The adjacent member within the same panel is considered to contribute no compressive strength. Utilizing tension only members makes very efficient use of the structural steel shape and will result in using the smallest members such as wind or seismic, is achieved by means of braces, which essentially work in tension or compression.

Steel – Cutting

Steel and weld metal may be thermally cut provided a smooth and regular surface, free from cracks and notches is obtained. All thermally cut surfaces shall be produced using a mechanically guided torch unless otherwise approved by the DCES. Thermal cut surfaces produced by a manually guided torch, when allowed, shall be smoothed by machining or grinding. In all thermal cutting, the cutting flame shall be adjusted and manipulated to avoid cutting beyond (inside) the prescribed lines. The roughness of thermal cut surfaces shall not exceed the American National Standards Institute surface roughness value of 1000 micro inches for material up to 4 inches thick and 2000 micro inches for material 4 inches to 8 inches thick, except, at the dead ends of members where there is no calculated stress, the roughness shall not exceed 2000 micro inches. Roughness exceeding these values and occasional notches or gouges no more than ¼ inch deep on otherwise satisfactory surfaces shall be removed by machining or grinding. Cut surfaces and edges shall be free of slag. Correction of discontinuities shall be faired to the oxygen cut surfaces with a slope not exceeding 1 in 10. Occasional notches or gouges that exceed ¼ inch shall be repaired by welding. The repair of notches or gouges over 7 /16 inch deep shall be referred to the DCES before repair. Welding repairs shall be made by suitably preparing the discontinuity, welding with an approved process after preheating .Minimum Preheat and Interpass Temperature and grinding is required to complete weld with smooth and flush with the adjacent surface to produce a workmanlike finish. All welded repairs to main material subject to tensile stress shall be tested by ultrasonic or radiographic inspection as determined by the DCES. Reentrant corners shall be filleted to a radius of not less than ¾ inch. On main material, carrying primary stress, a 2 inch [50 mm] minimum radius shall be provided wherever possible. The radius and its contiguous cuts shall meet without offset or cutting past the point of tangency. The surface to be welded must be adequately smooth, uniform, and free from fins, tears, cracks and other discontinuities which would adversely affect the quality or strength of the weld. Surfaces to be welded and surfaces adjacent to a weld must also be free of loose or thick scale, slag, rust, moisture, grease and other foreign material that will prevent proper welding or produce objectionable fumes. Mill scale capable of vigorous wire brushing, a thin rust inhibitive coating, or a compound may remain except that all mill scale shall be removed from the surfaces on which flange-to- web welds are to be made by any of the approved welding processes.

Surface Properties of Steel

Stainless steels have versatile mechanical properties. The corrosion resistivity makes steel unique among the engineering materials. As such the corrosion rate of ferritic stainless steels decreases drastically within a narrow concentration interval making the transition from iron-type to non-corrosive behaviour quite abrupt. The type of the oxide layer formed on the surface depends on the oxygen pressure, temperature and the alloy compositions in the vicinity of the surface. Later on, the oxidation process assumes transport of metal and oxygen ions through the initially formed oxide scale. The ion transport is controlled by diffusion, which in turn is determined by the defect structure of the oxide layer. The high mobility of Fe in Fe oxides, especially in FeO, which is the dominant oxide component on pure iron above 570°C, explains the corrosive nature of Fe. The passivity in Fe-Cr, on the other hand, is attributed to a stable Cr-rich oxide scale. Above the critical concentration, a pure chromium layer is formed on the surface which effectively blocks the ion diffusion across the oxide scale. The lack of Cr at the surface of Ferric alloys is a direct consequence of the anomalous mixing of Fe and Cr at low Cr concentrations, which in its turn has a magnetic origin. This finding has important implication in modern materials science as it offers additional rich perspectives in the optimization of high-performance steel grades. Chromium oxide gives good corrosion protection at usual operating temperatures but since Cr forms volatile compounds at high temperature the corrosion protection at elevated temperatures requires the more stable Al oxide scales on the alloy surface. The Cr2O3 scale is protective up to 1000-1100 °C whereas Al2O3 scales up to 1400 °C. Unfortunately, for most of the Fe alloy applications the straightforward procedure to improve high-temperature corrosion resistance by increasing the Al content in bulk, is not an acceptable solution. This is because the high Al content makes Fe-Al alloys brittle which poses a natural upper bound for the Al content in these alloys regarding most of the applications. Fortunately, the additional alloying of Fe-Al with Cr boosts the formation of the Al oxide scale on the surface up to such a level that the Al content in bulk can be kept within the acceptable limits regarding the required mechanical properties of the alloy. This phenomenon, called the third element effect, is still considered a phenomenon without a generally accepted explanation.

Precast elements

Production of precast elements is done in casting yard. Moulds of adequate stiffness have been used and installed as per issued GFC drawings. The sequence of activities involved in the production of precast elements is as follows:

1. Mould cleaning and preparation

2. Shuttering/assembling the mould components

3. Fixing of rebars / cast-in-fittings

4. Pre-concrete check

5. Concreting

6. Curing

7. Demoulding

8. Final inspection

9. Stacking

Stacking of Precast Elements. Lifting and handling of precast elements have been done using gantry cranes and other approved lifting devices. Number and location of lifting points have been decided such that the ‘handling stresses’ were always within allowable limits. For members having unsymmetrical geometry or projecting sections required supplementary lifting point. Precast elements such as precast slabs, beams, spandrels and staircase landing elements were stacked horizontally and supported with strips of wood or batten across the full width of the bearing points. Precast walls, column-beam-wall panels shall be stacked in vertical position supporting their self-weight using racks. Precast elements were transported to the site on a flatbed or low bed trailers. The delivery of precast elements was planned according to the erection sequence to avoid or minimize unnecessary handling or storage at the site. The precast elements are being loaded using gantry crane of 10 to 15 ton of varying capacity and delivered to the site with proper supports, frames and cushioning to prevent transit damages. The elements were delivered in a manner in which they can be lifted directly and erected without much change in the orientation or Sequence. Average 136 numbers of elements were to be transported per day to the site with the help of around 10 numbers of trailers.

Before the erection of precast elements, the following preparatory works shall be carried out to achieve efficient and quality installation.

1. Site accessibility shall be checked for the delivery of precast elements.

2. Check whether the right element is dispatched or not.

3. Visual inspection shall be done to check the concrete finishes and damages if any.

4. Crane shall be checked for its working clearance for hoisting the precast element.

5. The elements shall be stored using “First In First Out” principle according to the erection sequence

During the erection of vertical members, the reference line and offset line was set out to determine the position of the element to be installed. Then shim plates or level pads were provided for setting the level of the elements. For external wall or column elements, a compressible form of the backer rod was fixed on the outer perimeter of the wall. The element was placed in the designated location and secured with diagonal props (push-pull jacks) and then check for the alignment. After installing the element, grouting work was carried out. Approved grouting material was prepared as per specifications and applied to seal the gaps along the bottom edge of the inner side of the element. This is a technique of post-tensioning; where elements are placed first and then by inserting the grout material, tension is imparted in the element to achieve the highest strength. Erection of horizontal element is similar to that of the vertical element; but the difference is, props were kept ready before bringing the horizontal element to the location, and levelling is done. Screed concrete of 60mm thickness was placed over the slab as specified in the drawings.

Weldability of Steel

The term weldability is the ability to obtain economic welds, which are good, crack-free and would meet all the requirements. What is of great importance is the chemistry and the structure of the base metal and the weld metal. The effects of heating and cooling associated with welding are experienced by the weld metal and the Heat Affected Zone (HAZ) of the core metal. The base metal surrounding the weld metal and the weld itself have unduly varying hardness distribution across a weld. The hardness in steel depends upon the rate at which steel is cooled near the fusion zone; the hardness is maximum due to the higher temperature at that location and also have the maximum rate of cooling. A higher value of hardness leads to cracks in HAZ or in the weld. There are chances of Cracks that might be formed during or after the welding process. Good design and standard welding procedure help minimise the cracking problem.

The main features that affect weld cracking during the welding processes are

Joint restraint that builds up high stress in the weld (convex or concave)

Carbon and alloy content

Cooling rate

Hydrogen and nitrogen absorption

The cracks in HAZ are mainly caused by high carbon content, hydrogen embrittlement and rate of cooling. In most of the steels, weld cracks become a problem as the thickness of the plate increases. Types of joints and welds By means of welding, it is possible to make continuous, load-bearing joints between the members of a structure.

A variety of joints is used in structural steelwork and they can be classified into four basic configurations are

  1. Lap joint

  2. Tee joint

  3. Butt joint

  4. Corner joint.

The weld defects detected during inspection are acceptable for structures. • For joints welded from both the surfaces, incomplete perception with thickness up to 5% of the parent metal thickness, but not surpassing 2 mm and the expansion, more than 500 mm can be accepted. The aggregate expansion of stain shall not be more than 200 mm per meter length of the joint. Incomplete perception and cracks are not permitted at or near the end or beginning of a joint.

For joints united from one side without backing strip, incomplete penetration with thickness up to 15% of parent metal thickness but not exceeding 3 mm at the root is permitted.

Slag inclusion located along the weld as a chain or unbroken line is allowed if their whole length does not exceed 200 mm per meter of the weld length. Size of the slag may also be considered.

Total of isolated gas pores and slag inclusion shall not exceed 5 in number per square centimetre of the weld.

The incomplete penetration, slag inclusion on pores located separately or as a chain shall not exceed 10% of metal thickness but not greater than 2 mm when welding is done from both the sides and 15% of metal thickness, but not vaster than 3 mm when welding is done from one side.

Structural steel

Structural steel has superior characteristics when compared with competing materials. In order to replace an area of steel in tension, an equivalent plain concrete area of about 200 units would be required. To compete with Structural Steel in construction, Reinforcing Steel needs to be added to it. The cracking of concrete in tension still cannot be avoided, which often encourages corrosion of reinforcement. In compression, one unit area of steel is the equivalent of 15-20 units of M20 concrete. A comparison shows that steel is at least 3.5 times more effective than concrete. In a compressive loading, there would be 8 times shortening of steel in concrete. Reinforcing steel has to prop up the plain concrete. In structures built of Structural Steel, accidental overloading does not usually lead to great havoc, as there are a considerable reserve strength and ductility. Steel may thus be regarded as a forgiving material whereas concrete structures under accidental overload may well suffer the catastrophic collapse of the whole structure. Repair and retrofit of steel members and their strengthening at a future date (for example, to take account of enhanced loading) are a lot easier than that of strengthened concrete members. The quality of steel-intensive construction is invariably superior when compared with all other competing systems (including concrete structures) thus ensuring enhanced durability. The quality control in construction at the site in India is poor. Structural Steel is recyclable and environment-friendly. Over 400 million tonnes of steel infrastructure and technology for the recycling of steel is very well organised. Steel is a material that can be easily recycled. The recycled, steel can change from one product to another without losing its quality. Steel can as easily turn up in precision blades for turbines or super strong suspension cables. Recycling of steel leads to protection of energy and primary resources and reduces waste. A steel building can be easily designed to allow disassembly or deconstruction at the end of their useful lives leading to many environmental and economic benefits; it can mean that steel components can be re-used in later buildings without the need for recycling, and the lack of proper usage of the energy used and the by-products from the steel production processes. Steel-intensive construction results in the least disturbance to the community in which the structure is located. The latest construction techniques developed in recent years with the help of steel-intensive solutions have been brought to effect that leads to the least disruption to traffic and minimises financial losses to the community and business. As such the initial cost of a concrete intensive structure may sometimes appear to be cheaper, in comparison to the equivalent steel-intensive structure, but it is a known fact that the total lifetime cost of gold is significantly higher. The usual belief that the concrete-intensive structure is cheaper is not based on verifiable facts! There is, therefore, no real cost advantage either.