14 - Reading Structural Steelwork Drawings

› Note 13 Level 1

32

Technical Technical Guidance Note

TheStructuralEngineer August 2012

Reading structural steelwork drawings Introduction

This Technical Guidance Note describes how drawings for structural steelwork are developed and read. They have their own unique set of rules and nomenclature and it is important for engineers to understand all of these rules in order to communicate and interpret the design of steelwork structures.

ICON LEGEND

W Applied practice

This guide is split into two sections; the first contains the information a designer of the steel elements provides, whilst the second contains the information a fabricator creates in order to manufacture and construct the steel structure. While one feeds into the other, the level of detail each set of information provides is very different, due primarily to the end result. One is informing the manufacture of the steelwork, while the other focusses on its installation.

Drawing principles When preparing general arrangements for steel framed structures a diagrammatic style should be applied. All beams and trusses are drawn as a single thick black line and bracing is shown as a dashed line when drawn in plan. All general arrangement drawings are to scale and any detail sections through the structure have all elements drawn as they would appear in reality. Also beams are generally viewed and labelled South to North/Down to Up and East to West/Right to Left.

Design intent general arrangement drawing protocol For building projects, the responsibility of the design and detailing of connections within a steel frame falls to the steel fabricator in most instances. This is not the case for civil engineering based projects however, as all of the design responsibility falls to the design engineer. The engineer/designer of the building structure must ensure that the fabrication drawings and connection calculations meet the design requirements that they have defined. It is for this reason that design general arrangements that

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Column size shown as depth, width and weight/m

W Drawing principles

W Further reading

W Web resources

Beam size shown as depth, width and weight/m Column size shown as depth, width and weight/m

Ultimate end reactions

Ultimate end reactions

N Figure 1

Plan on steel beam with reactions - as shown in a design drawing

are developed by the design engineer do not include any details on the sizing of components such as bolts, fin plates and welds that make up the connections. They do however show the ultimate reaction forces that occur at each connection within the steel frame; such as shear, bending moment, axial and torsion forces. It is also required of the design engineer to pass on the details of the tie forces that occur at all connections. This is to ensure the robustness requirements (as stipulated in part A3 of the Building Regulations of England) are fulfilled. Details of the form the connection should take are also described within the design documentation. This information is provided

to the fabricator so that they can develop the design of the connections. In some instances the design of the elements themselves become the responsibility of the steel fabricator and in such cases the design engineer provides limits on sizes and form of the steel elements. In other cases the design responsibility for all elements of the steel structure falls to the design engineer, with the fabricator following their specified member sizes and connection details exactly. More detail on apportioning design responsibility for steel frame structures can be found in the BCSA & SCI publication: National Structural Steelwork Specification (5th Ed.) - Tables 1.2A, 1.2B & 1.2C.

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Figure 1 shows a plan of a beam with its reactions. The labelling system used for steel elements is dependent upon the type of steel section. For traditional open sections such as I beams and H columns the protocol is as follows: Depth in mm x Width in mm x Weight per metre length in kg/m Note that the dimensions given for these sections are not the actual dimensions of the element but the serial size. The actual dimensions vary depending on the serial weight per metre stated in the label. For example a 203x203x86 UC is a 222mm deep universal column section where as a 203x203x52 UC is a 206mm deep universal column section, yet they are the same serial size in terms of labelling. For angle and hollow sections the label does accurately state the actual dimensions of the section along with the thickness of the plate it is made from, thus: Depth in mm x Width in mm x Thickness in mm Therefore a 120x120x12mm RSA is a rolled steel angle that is 120mm deep and 120mm wide and has a thickness of 12mm. With regard to hollow sections, a 150x100x5 RHS is a rectangular hollow section with a depth of 150mm, a width of 100mm and a plate thickness of 5mm. Figure 2 is a general arrangement drawing that has examples of all of the elements described previously. Note how the beams are drawn with gaps at the end as they connect to the columns. This indicates that they are separate elements from the one they are connecting to. An alternative method of labelling elements within a steel structure is the use of a table of member sizes that correspond to a code. This code is used as a label instead of the actual beam size on the drawings. This is done in order to reduce clutter on the drawings, making them easier to read. It is important to note that the connection detail shown in Figure 3 does not have any information regarding the bolt, cleat, and weld sizing. All of these elements typically fall under the design responsibility of the fabricator and any detail developed by the design engineer must not be specific on these components that make up connections.

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N Figure 2 Example general arrangement plan of a steel structure

As well as connection forces and their design intent, it is required that the design engineer specifies the steel grades and sub-grades from which the structural steelwork is to be formed. For building structures these are typically S275J0 or S355J0 for example. Drawings should also highlight any assumptions regarding the sequence of erection, temporary stability and other unique aspects that would affect construction. Furthermore, there should also be an indication of any shear studs required for composite beams that are designed to work in conjunction with the floor slab. Finally, drawings should show foundations indicatively (and note them as such) so that the fabricator understands the interface between the steel superstructure and the sub-structure that supports it. This will allow them to complete the design of the base plate to columns along with their corresponding holding down bolts. There are some terms used on steelwork general arrangements that designate the relevant level of the elements shown on them. The term TOS stands for Top of Steel and is the level at which the top surface of the steel is elevated. SSL is Structural Slab Level i.e. the level at which the floor slab is located. FFL is the Finish Floor Level and is the upper most level of the overall floor structure. It is only indicated on steelwork general arrangements as a navigational aid.

N Figure 3 Design intent detail of beam to column connection

"Note that the dimensions for these sections are not the actual dimensions of the element but the serial size"

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› Note 13 Level 1

34

Technical Technical Guidance Note

TheStructuralEngineer August 2012

Eurocode 0.

Applied practice

The applicable codes of practice for structural steelwork drawings are as follows: N Figure 4 Plan on steel beam as shown in a fabrication general arrangement

S Figure 5 Beam from Fig. 1 drawn as a steel fabrication element

BS EN 1993-1-1: Eurocode 2: Design of Steel Structures. Part 1-1 General Rules and Rules for Buildings BS EN 1993-1-1: Eurocode 2: UK National Annex to Design of Steel Structures. Part 1-1 General Rules and Rules for Buildings BS 4 Part 1:2005: Structural steel sections — Part 1: Specification for hot-rolled Sections BS EN 22553:1995: Welded, brazed and soldered joints — Symbolic representation on drawings

Glossary and further reading Design general arrangement – Fully dimensioned drawing of the steel framed structure and other structural elements. Steel fabrication general arrangement drawing protocol General arrangements developed by steel fabricators are based on the design general arrangements and only show the steelwork elements of the structure. The beams are drawn to scale in plan and rather than label the size of elements in the frame the fabricators instead give them a unique marker. These markers are used to designate the components of the structure, which the fabricator refers to when constructing the frame. Figure 4 is an example of the same beam shown in Fig. 1 as it would appear in a steel fabrication general arrangement.

Steel fabrication shop drawings Every element in a steel frame has a fabrication drawing produced for it. These show what needs to be done to the steel element in order for it to be installed within the frame. This includes bolt holes, fin plates, welds and stiffeners that are developed based on the connection design that is typically carried out by the fabricator. In some instances these drawings show the connection data drawn to scale, but the length of the element is not. Instead the element’s length is defined but highlighted as being not to scale. This is a form of shorthand so that the elements can be drawn to a larger and easier to read scale without having to

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draw the whole length of them. Figure 5 is a drawing of the steel beam first seen in Fig. 1 drawn as a steel fabrication element. Welding has a unique set of symbols that are defined in BS EN 22553:1995 Welded, brazed and soldered joints - Symbolic representation on drawings. This system is used throughout shop drawings to indicate the type of weld, its thickness, length and location. Figure 6 is an annotated example of one of these weld labels. The introduction of CAD packages has had a significant impact on how steel members are presented. In many cases viewers are used to inspect 3D computer models, which enable the design engineer to pan and zoom around the model and view the properties of each member, including length, section size, steel grade and connection details. The basic concepts of labelling elements however remain the same, no matter how they are presented.

N Figure 6 Example of a fillet weld label

Fabrication general arrangement – Fully dimensioned drawing of the steel elements of a structure, developed by the fabricator.

Further Reading BCSA & SCI (2007) 203/07 National Structural Steelwork Specification for Building Construction (5th ed) London:BCSA Simmons, C.H., Maguire, D.E. and Phelps, N.: (2012) Manual of Engineering Drawing: Technical Product Specification and Documentation to British and International Standards 4th ed. Oxford: Butterworth-Heinemann Eurocode 0.

Web resources

Tata Steel Interactive ‘Blue Book’: www.tatasteelconstruction.com/en/design_ guidance/the_blue_book/

Throat Type of thickness weld

Thickness dimension

Design intent – Design information provided by the design engineer to the fabricator to indicate the preferred form of the structural element.

Length of weld

The British Constructional Steelwork Association: www.steelconstruction.org/ The Institution of Structural Engineers library: www.istructe.org/resources-centre/library

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