This page provides an overview of structural design principles, including the behaviour of beams, columns and frames, along with loading conditions, resistance and classification used in engineering structures.
The information is intended to support engineers in understanding structural behaviour, applying design standards and analysing loads and stresses in construction and mechanical systems.
Introduction..... Formwork..... Standards..... Symbols..... Properties..... Methodology..... Design working life..... Resistance..... Actions..... Classification..... Frames..... Connections.....
The building structures pages have been added over the six months to Dec. 2012. They are very much work-in-progress and I will be updating them on a regular basis over the next six months.
This section includes outline notes on the design of structures. The notes are general in nature and are included to provide understanding of principles involved. It does not provide sufficient information to allow detailed design to be completed. For detail design of structures the relevant standards should be used and also reputable reference documents. In fact for all but the most simplest structures, design generally requires use of computer systems.
The information on this page is, as of June 2012 very limited, however links are provided in the text and at the foot of the page which include very detailed information sufficient for detail design. This page also includes a list of the current relevant standards.
This website includes notes on the elastic design of beams, columns and frames in the pages linked below.
| Statics | Struts | Beam Index |
Eurocode 3: Design of Steel Structures
| Code Reference Number | Title |
| BS EN 1993-1-1:2005 | Eurocode 3 :General rules and rules for buildings..Replaces BS 449-2, BS 5400-3 and BS 5950-1 |
| BS EN 1993-1-2:2005 | Eurocode 3 :General rules. Structural fire design ..Replaces BS 5950-8 |
| BS EN 1993-1-3:2006 | Eurocode 3 :General rules. Supplementary rules for cold-formed members and sheeting .. Replaces BS 5950-5, BS 5950-6 and BS 5950-9 |
| BS EN 1993-1-4:2006 | Eurocode 3 :General rules. Supplementary rules for stainless steels |
| BS EN 1993-1-5:2006 | Eurocode 3 :Plated structural elements...Replaces BS 449-2, BS 5400-3 and BS 5950-1 |
| BS EN 1993-1-6:2007 | Eurocode 3 :Strength and stability of shell structures |
| BS EN 1993-1-7:2007 | Eurocode 3 :Plated structures subject to out of plane loading |
| BS EN 1993-1-8:2005 | Eurocode 3 :Design of joints ..Replaces BS 449-2, BS 4604-1, BS 4604-2, BS 5400-3 and BS 5950-1 |
| BS EN 1993-1-9:2005 | Eurocode 3 :Fatigue...Replaces BS 5400-10 and BS 7608 |
| BS EN 1993-1-10:2005 | Eurocode 3 :Material toughness and through-thickness properties...Replaces BS 449-2, BS 5400-3 and BS 5950-1 |
| BS EN 1993-1-11:2006 | Eurocode 3 :Design of structures with tension components |
| BS EN 1993-1-12:2007 | Eurocode 3 :Additional rules for the extension of EN 1993 up to steel grades S 700 |
| BS EN 1993-2:2006 | Eurocode 3 :Steel bridges ..Replaces BS 5400-3 |
| BS EN 1993-3-1:2006 | Eurocode 3 :Towers, masts and chimneys. Towers and masts ...Replaces BS 8100-1, BS 8100-2, BS 8100-3 and BS 8100-4 |
| BS EN 1993-3-2:2006 | Eurocode 3 :Towers, masts and chimneys. Chimneys ...Replaces BS 4076 |
| BS EN 1993-4-1:2007 | Eurocode 3 :Silos |
| BS EN 1993-4-2:2007 | Eurocode 3 :Tanks |
| BS EN 1993-4-3:2007 | Eurocode 3 :Pipelines |
| BS EN 1993-5:2007 | Eurocode 3 :Piling...Replaces BS 449-2 and BS 5950-1 |
| BS EN 1993-6:2007 | Eurocode 3 :Crane supporting structures ...Replaces BS 2853, BS 449-2 and BS 5950-1 |
|
x-x = axis along member y-y = axis of a cross section z-z = axis of a cross section b = length of cros ssection h = depth of cros ssection L = member length a d = design value of a geometric property f y =Yield strength f u = Ultimate strength εu = Ultimate strain ν = Poissen's ratio. E = Modulus of elasticity G = Shear Modulus Resistance symbols... R k = Characteristic value for material resistance R d = Design value for material resistance Xd = design value of material property Xk = characteristic value of material property γM Global partial factor for a particular resistance η = Conversion factor for a particular resistance |
Action symbols.. Fk = Characteristic value of action Fd = Design value of action Gk = Characteristic value of permanent action Gd = Design value of permanent action Qk = Characteristic value of variable/imposed action(single value) γG = Partial factor for permanent action γQ = Partial factor for variable action ψ0 = Factor for combination value of variable action ψ1 = Factor for frequent value of variable action ψ2 = Factor for quasi-permanent value of variable action ε = Combination factor for permanent actions ξ = Reduction factor for unfavourable permanent actions |
The material properties are found in the relevant steel standards e.g. BS ISO 10025. The following relationships apply.
fy = ReH
fu = Rm
BS EN 1993 -1 clause 3.2.2 identifies the recommended ductility requirements for structural steel in terms of the ratio of fu / fy,the elongation at failure, and the ultimate strain
fu / fy >= 1,10
elongation at failure should be not less than 15%
Ultimate strain εu >= εu / εy...... ( The yield strain εy = fy /E)
BS EN 1993 -1 clause 3.2.6 identifies the recommended design values for structural steel coefficient as follows
Modulus of elasticity E = 210 000 N/mm2
Shear Modulus G = E / 2( 1 + ν ) = approx 81 000 N/mm2
Poissen's ratio in elastic range ν = 0,3
Coefficient of linear expansion α = 12 x 10 -6 per deg. K ( for T <= 100o C)
The intended design life of the construction should be identified at the initial stage of the design process
table of design lives of buildings in accordance with BS EN 1990 clause 2,3
| Design working life category | Indicative design working life (years> |
Examples |
| 1 | 10 | Temporary structures |
| 2 | 10 to 25 | Replacable structures e.g. Gantry girders, bearings |
| 3 | 15 to 30 | Agricultural Builidng and similar structures |
| 4 | 50 | Buildings and other common structures |
| 5 | 100 | Monumental structures , bridges, civil engineering structures |
Notes on the resistance to of a structure are provided on webpage Eurocodes Intro A variation on the equations as applicable to steel structures provided below.
Xk = characteric value of material property
Rk = characteristic value of the particular resistance determined using the characteristic values for the material
properties and dimensions.
γM = Global partial factor for a particular resistance
η = Conversion factor for a particular resistance,taking into account factors such as
ambient conditions, scale factors etc.
| Resistance of bolts | γM2 = 1.25 | ||
| Resistance of welds | γM2 = 1.25 | ||
| Resistance of plates in bearing | γM2 = 1.25 | ||
| Slip resistance at ULS | γM3 = 1.25 | ||
| Slip resistsnce at SLS | γM3.ser = 1.10 | ||
| Preload of high strength bolts | γM7 = 1.10 |
| Beams | Columns | Frames | Shear Panels |
| Rings | Arches | space Frames | Plates |
| Thin Shells | Tension Structures | Inflated Structures |
A frame is part of, or all of a structure , comprising of an
assembly of directly connected structural elements, designed to act together to resist
load. This relates to moment resisting frames and triangulated frames and
covers both plane frames and three-dimensional frames.
A sub-frame is part of a larger frame which is treated as and isolated frame for the purpose
of structural analysis.
Frames types
Trusses
A truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in forces in the members which are either tensile (in a tie member) or compressive forces (In a strut member. All of the nodes in a truss are assumed to be pin joints which do not transmit moments. Trusses are easily separated into two groups : pitched trusses as used for building roofs and parallel trusses which are often used for spanning e.g bridges. The figures below illustrate ,very crudely, a selection of truss designs. These are shown as planar constructions . The designs are also relevant to 3 D constructions " Space Frames".
Selection of Pitched Trusses
Selection of Parallel Trusses
Portal Frames
Many single story buildings are constructed from basic portal frames. Portal frames are generally two
dimensional rigid frames based on support columns and traversing beams. The frames have the basic characteristics of a
rigid joint between column and beam. Portal frames are generally constructed from Steel or reinforced concrete but timber
is also used.
Portal frames are an efficient construction technique for use for wide span buildings. Portal frame construction is
therefore typically seen in leisure centres, warehouses, barns and other places where large, open spaces are required at low cost.
Generally portal frames are used for single story buildings but they can be used for low rise buildings with several floors where they
can be economic if the floors do not span right across the building (in these circumstances a skeleton frame,
with internal columns, would be a more economic choice). A typical configuration might be where there is office space
built against one wall of a warehouse.
The forces involved in three hinged portal frame can generally be easily resolved using statics. The problem is then broken down to determination of the shear forces and the bending moments for the two individual beams making up the portal frame.
Two hinged and hingeless portal frames are classified as statically indeterminate structures and conditions other than statics conditions have to be used to calculate the end reactions and moments. Consider a two hinge portal frame as shown below. Plastic hinges are formed in the eaves and near the apex. A very preliminary method of estimating the horizontal base loads and the design moments in the rafters and stanchions involves using the graphs as shown below.
The load W = the vertical rafter load per metre run
The Horizontal base reaction can be obtained using the graph below
The design moment for the stanchion be obtained using the graph below
The design moment for the rafter be obtained using the graph below
Connections between structural elements is one of the most important areas of design structural steelwork. There are three principle types of connection .
1) Simple connections transmit negligible bending moments across the joint . This joint allow the
beams to rotate relative to each other . They are effectively pin joints.
2) Continuous connections are designed to transmit shear forces and bending moments across the joint. The connection
should have sufficient stiffness to allow design using either elastic or plastic analysis.
3)Semi-continuous connections are designed to transmit the shear forces and a proportion of the bending moment. These provide a partial
restraint to the beam rotation with a relatively less complicated structural joint. This is often a more complicated arrangement
to design.
