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Structures Index

Structural Design

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.

Introduction

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.   For free software programme "Framework" which allows design of simple structures even in accordance with the relevant Eurocoodes ref Mechanical programs.

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





Relevant Standards

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



Symbols
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







Material Properties for Steel Structures

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)






Design methodology as required by BS EN 1993






Design Life

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 10Temporary structures
2 10 to 25Replacable structures e.g. Gantry girders, bearings
3 15 to 30Agricultural Builidng and similar structures
4 50Buildings and other common structures
5 100Monumental structures , bridges, civil engineering structures





Design Resistance

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 bearingγM2 = 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

Notes on Resistance are found on webpage introduction to eurocodes. Eurocodes Introduction








Design Actions

For steel structures the design effect of actions is ( Ed ) is defined on webpage Eurocodes Intro.

The general equation for the Effect of actions should be

The part of the equation inside the brackets represents the combination of permanent and variable actions
the applicable equations for the combination of actions as referenced in BS EN 1990 are 6.10a and 6.10b



Notes on Actions (imposed forces) are found on webpage introduction to eurocodes. Eurocodes Introduction




Classifying Structural elements

The simplest classifications of structures is based on the method of analysis.    Structures can be either statically determinate or statically indeterminate. Statically determinate structures can be analysed using the principles of simple statics and statically indeterminate structures can not.

Within these broad classifications ,structures can be broken subdivided into smaller groups by their configuration or use.    The following groups typify the various classification of structural element.

Beams ColumnsFrames Shear Panels
Rings Archesspace Frames Plates
Thin Shells Tension StructuresInflated Structures







Frames definition

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






Connection types

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.

Useful relevant Links
  1. Mechanical programs....Includes Framework download - an advanced struct. analysis program
  2. Steel Building Design: Concise Eurocodes... document incuding introduction to actions :118 page document
  3. Corus Construction... This site provides all the information you need for structural design
  4. MW tech...-> Mechanical Engineering -> Material Sizes
  5. xcalcs... Section information and Calculations
  6. Europrofil...Detailed Sections Based on European Codes (german) -using via google -> translating function helps
  7. Technouk...Various Spreadsheet Downloads- including comprehensive section properties tables
  8. Cadoss...A site dedicated to structural design - includes downloads and CD's
  9. RSSB Design of Railway Structures to Structural Eurocodes.... A detail document download including


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Last Updated 05/10/2012