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.

Eurocode 3: Design of Steel Structures

The material properties are found in the relevant steel standards e.g. BS ISO 10025. The following relationships apply.

f_y = R_eH
f_u = R_m

BS EN 1993 -1 clause 3.2.2 identifies the recommended ductility requirements for structural steel in terms of the ratio of f_u / f_y,the elongation at failure, and the ultimate strain

f_u / f_y >= 1,10
elongation at failure should be not less than 15%
Ultimate strain ε_u >= ε_u / ε_y...... ( The yield strain ε_y = f_y /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 <= 100^o 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

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.

X_k = characteric value of material property
R_k = characteristic value of the particular resistance determined using the characteristic values for the material properties and dimensions.
γ_M = 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.

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

For steel structures the design effect of actions is ( E_d ) 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

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.

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.