design for structural steel work for frame industrial building

8/9/2019 Design for Structural Steel Work for Frame Industrial Building

1/100

The

SteelConstruction

Institute

S

Design

of Structural Steelwork

Lattice Framed

Industrial

Building

(Revised Edition)

mit

deutscher

Zusammenfassung

_______________________

avec résumé

français

—

=

___________

This document

con resumen

español

___________

contains

100

pages

con sommario

taliano

-

Institutde

a

Construction

Métallique

InstitutfürStahlbau IstitutodiCostruzioni inAcciaio Instituto de IaConstrucciOnMetálica

/


2/100


3/100

SCI PUBLICATION

028

Design

of

Structural Steelwork

Lattice

Framed Industrial

Building

(Revised

Edition)

Entwurf

elnes Stah/bau-Gebãudes

-

G/tterahmen /ndustriegebãude

Dimensionnement d'/mmeublesa structure

meta/llque

-

bétiment

industriel en cadre

et

tre /I/s

Progettazione

dlEd/f/cl in Accialo:Ed/f/cl Industrial! Inte/alat/ a

Tra/lcclo

Pro

yecto

de Ed/f/c/os con Estructura de Acero.

Ed/f/do /ndustr/al

en

Ce/os/a

CSOUTHCOMBE

BSc(Eng), MSc(Eng), CEng,

MICE

ISBN

1

870004 83

3

British

Library

Cataloguing

in

Publication Data

A

atalogue

record for this book is available from the British

Library

©

The Steel Construction Institute

1993

The Steel

Construction Institute

Silwood

Park

Ascot

Berkshire SL5

7QN

Telephone:

0344 23345

Fax:

0344 22944


4/100

FOREWORD

This

publication

is a revised editionof the

original

text written

by

Mr W

Bates

and

first

published

in

1983.

Its

purpose

is

to

aid the

education

of

undergraduate students

in

Engineeringby

providing

sample

calculations

for

a

ypical

industrial

building capable

of

future

extension.

The revision was made

necessary

by

changes

in

design

Codes

and current

practice

over the

past

decade.

For their

helpful

contributions

regardingdesign,

fabrication

and

the erection

process,

the

author

is indebted to:

Mr. A. Curnow

(Blight

and White

Limited,

Plymouth)

Mr. R. Fox

(F.

Parkin and Son

Ltd.,

Exeter)

Mr.

P.

Marozinski

Conder

Limited,

Winchester)

11


5/100

CONTENTS

Page

FOREWORD

U

SUMMARY

v

1. INTRODUCTION 1

2.

SCOPE 2

3.

STANDARDS

AND

CODES OF PRACTICE

4

3.1 British Standard

5950

-

Structural use of steelwork

in

building

4

3.2 BS 5502

-

Buildings

and structures for

agriculture

4

3.3 BS 6399: Part

1: 1

984

-

Design Loading

for

Buildings

4

3.4

BS

6399: Part

3: 1

988

-

Code

of

practice

for

imposed

roof

oads

4

3.5

CP3:

Chapter

V:

Part 2: 1972-Wind Loads 4

3.6

Statutory

regulations

5

3.7

National structural steelwork

specification

for

building

construction

(2nd Edition) 5

3.8

Quality

assurance

5

4. BUILDING FORM

6

4.1

General 6

4.2 Low

pitch

roofs 6

5.

LATTICE FRAMED ROOFS 8

5.1

Simple orms

8

5.2

More

complex

forms 10

5.3

Cladding

1

2

5.4 Purlins 13

5.5 Side rails 14

6. CONCEPTUAL

DESIGN

16

7. PRINCIPLESOF DESIGN 19

7.1

Purlins and side rails 19

7.2

Lattice framed roof

girders

19

7.3 Stanchions 1 9

7.4

Bracing

21

7.5 Connections 21

111


6/100

CONTENTS

-

Continued

Page

8.

EXAMPLE

-

DESIGN

BRIEF

AND

APPROACH

24

8.1 Brief

24

8.2

Cladding

24

9.

DESIGN OF STEELWORK

27

9.1

Loading

27

9.2 Assessmentof roof

load

27

9.3 Assessmentof

wind

load

on structure 28

9.4

Design

of

purlins

31

9.5

Design

of

main roof

frame

36

9.6

Preliminary

calculations

37

9.7

Loading

Cases

(for

characteristic

oads)

40

9.8

Analyses

40

10.

FINAL DESIGN

49

10.1

Top

boom

49

10.2 Bottom

boom

51

10.3

Internal

members

53

10.4

Comparison

of

member sizes

54

10.5

Column

design

-

members 1 to

4

and 5

55

10.6

Gable

steelwork

61

10.7

Bracing

67

10.8 Column Base (Reference

1.

Clause

4.13)

70

10.9

Foundation

73

11.

ALTERNATIVE

FRAME ANALYSIS

75

12. JOINT

DESIGN

78

12.1

Application

limit check list

78

12.2 Joint welds

81

13.

FINAL FRAME LAYOUT

84

REFERENCES

87

BIBLIOGRAPHY

89

iv


7/100

SUMMARY

Design

of

structural

steelwork

-

Lattice framed ndustrial

building

The

designer

f

single storey

buildings

for

commercial

and

industrial

use will

consider a

number

of

possible

solutions. A

decisionhas to

be made

regarding

cladding,

structural form

and material.

This

publication

illustrates

for the

benefit of

students,

the

many

factors which

influence the final choiceofa suitable

design.

Consideration

is

given

to a

variety

of

building

forms as

well

as to the choice

of

cladding

and

its

supporting

element at

the

conceptual

design stage;

other factors

influencing

the

design

are

related to

fabrication,

transport

and

erection.

A structural steelwork

frame

incorporating

solid

web

beams

for

columns

and

a latticed

structure

for

the

roof,

is

chosen and

full

design

details

worked out.

The

detailed

design

ofa

building

30 m

wide,

48 m

long

x 6 m to

eaves

is

provided

as

an

illustration.

The solutionconsiders the main

loading

calculations and members

initially.

A detailed

analysis

is carried

out and

checks

are made

of

all

members,

the latticed roof

being

formedof

rectangular

hollow

section.

Typical

joints

and the

foundation

are

designed.

Entwurf

eines Stahlbau-Gebäudes

-

Gitterahmen

Industriegebäude

Zusammenfassung

Der

Konstrukteuer

eines

eingeschossigen

Handels

-

oder Industrie-Gebaudes ird eine

Reihe

moglicher

LOsungen

in Berracht

ziehen.

Entscheidungen

mQssen

getroffen

werden hinsichteich

Verkleidung, Formgebung

und

zu

verwendender

Werkstoffe.

Diese

Veroffentlichung

illustriert

zumNutzen von Studenten die vielen

Faktoren,

die die

endgtlltige

Wahieines

geeigneten

Entwurfsbeeinflussen.

Bei der

Konzeptentwickiung

werden verschiedene Gebäude-Formen als

auch eine

Auswahl von

Verkleidungen

und

ihre

Befestigungs

-

Elemente

betrachiet;andere Fakioren,die

den

Entwuif

beeinflussen, betreffen ilerstellung,

Transport

und

Errichtung.

Em Stahlbaurahmen

mit soliden

Ste

gträgern

fir

die

Stlitzen

und etne

GitterstrukiurftJr

das

Dach wird

gewahit,

wozu alle

Entwurfs-Einzelheiten ausgearbeiter

wurden.

Als illustration

1st

derdetaillierte

Enlwurfeines

Gebäudes

mit 30m Breite und 48 m

Lange,

sowie

6

m

bis

zur

Unterkante

des Daches

dargesteilt.

Belder

LOsung

wurden

die

wesentlichen

Lastberechnungen

der

Glieder im

Ausgangszustand

beracksichtigt.

Eine deraillicerte

Analyse wurde durchgefilhrr sowie

alle

Glieder

aberprtlft;

das

Rahmendach

wird aus

rechteckigen Hohlquerschnirren gebildet. Typische Verbindungen

und die

Grtindung

sind

dargesrelir.


8/100

Dimensionnement d'immeubles a

structure

métallique

-

bãtiment

industriel

en cadre et

treitlis

Résumé

Le

projeteur

d

immeubles,

a un

seul

niveau,

pour

usage

industriel

et commercial

peut

envisager

de

nombreuses solutions constructives,

une

decision

doit

être

prise

concernant

la

forme

structurale,

les

parios

et le

matériau.

Cette

publication

discute,

a 1 intention des

étudiants,

les

nombreuxfacteurs

qui

influencent

le

choix d un bon dimensionnement.

On considèreune

grande

variétE

de

ormes

de

bãtiments ainsi

que

le

choix des

parois

et

des

éléments

qui

les

supportent,

dans

le cadre

de

I

etape

de

conception

du

bãtiment. D autres

facteurs

qui

influencent

le

dimensionnement et

qui

sont

relatfs

a

la

abrication,

au

transport

et

au

montage,

sont

egalement

discutés.

Une structure

en acier

comportantdes

colonnes

en

prof/set

une toiture en treillis,

est

choisie

et étudiée

en

detail.

Le dimensionnement détaillC

d un bâtiment

de 30

m

de

large,

48 m de

long

et 6

m

sous

la

toiture

est

donnC

comme illustration.

La solution

comporte

une

analyse

détai!lée et une

verfi

ation

de

tous

les

Cléments,

le treillis

de

toiture étant rCalisC en

profils

creux

rectangulaires;

certains

assemblages

ainsi

que

les

fondations

sont

egalement

étudiés.

Progettazione

di

Edilici in Acciaio: Edifici Industriali Intelaiati

a Traliccio

Sommario

Nella

pro ettazione

di

edfici

monopiano

ad

usocommerciale

e industrialedevono essere

esaminate

dirvese

possibili

so!uzioni.

E'

necessario

operare

Ia

scelta del

rivestimento,

della

struttura

portante

e del materiale.

Questapubblicazione resenta,

a

beneficio degli

studenti,

tutti

quei

fattori

che

infiuenzano

la scelta

finale

in

vista

di

una

adeguatapro ettazione.

Per lafase

preliminare

di

progettazione

viene

presa

in considerazione

la varieta'

delle

tipologie

strutturali,

!a

scelta del rivestimento

e

dci suoi

elementi

di

collegamento,

a/tn

fattori

che

influenzano

ii

pro etto

sono

que/li

relativi

alla

lavorazione,

a!

trasporto

ed

al

montaggio.

Si il/ustra in

particolare,

sviluppando

tutti

i

dettagli

relativi

al

pro etto,

un

edfici

intelaiato in

acciaio,

formato

da colonne

ad anima

piena

e da elementi di

copertura

realizzati con una

struttura a traliccio.

A

titolo

di

esempio

viene

presentata

la

pro ettazione dettagliata

di

un

edficio

alto

6 metri con

dimensioni

in

pianta

di

30

metri

di

larghezza

e

48

metni

di

lunghezza.

Sono

presentati

I

principali

calcoli relativiai carichi

ed

al

predimensionamento.

L

analisi

dettagliata

e'

seguita

dalla verfica

di

tutti gli

elementi

portanti.

In

particolare

Ia

struttura

a

traliccio

onizzontale

e'

formata

da

sezioni

rettangolari

cave.

Vengono

inollre

progettati

alcuni

giunti tipici

e le

fondazioni.

vi


9/100


10/100

1. INTRODUCTION

In

general

the basic

brief or

the

design

of

he

majority

of

single

storey

buildings

for

industrial

and commercial use

is to

provide,

for

the

client,

a

structure

which

has

no

internal columns.

If

some columns

are

essential

the

numbershould

be

limited.

Thus,

in

principle,

the

requirement

is

for the

construction

of

four walls and

a

roof for

a

single

or

multi

bay

structure. The walls

can

be

formed

of

differentmaterials

e.g.

steel columns with

cladding

which

may

be of

profiled

or

plain sheet, precast concrete,

or

masonry

load

bearing

walls etc.

The

designer

will

generally

consider

for

the

roof

a

system

of

beams

or

latticed frameworks

in

structural steel

to

support

the roof

cladding.

Solid webbeams will make use

of

universalbeam sections.

The

use of

light

atticedframeworks

for

the

roofof

an industrial

buildingprovides

a

neat,

efficientstructurewhich

frequently

satisfies architectural

requirements.

The

design

of

he

steelwork

s

simple.

Modernfabrication

systems

and erection

procedures

makethese

structural forms economic.

This

is

particularlyapparent

when

it is

appreciated

how

many

industrial

buildings today

employ

latticed

roof

framing

and how

many

makers

of

standard

buildings,

as well as

suppliers

of

industrialised

buildingsystems,

make

useof

his

type

of

framing

in

preference

o

solid web

beamconstruction.

The

purpose

of

his

publication

isto

discuss

he

many

factors which

can

influence

the decision

making

process

and can lead

to

adopting

atticed framework construction. Alternative

design

solutions are then illustrated

by

means

ofa

practical example.

1


11/100

2.

SCOPE

The

scope

of

he

publication

is

mainly

restricted to

plane

frame structures. Other

forms,

such

as

space

frames,

are notconsidered in detail.

Various

types

of

steel

sections

are

used in the

construction

of

he

components

for

this type

of

structure,

viz, hotrolled structural

shapes

such

as universal

beams,

universal

columns,

angles,

structural hollow

sections and cold formed

sections,

etc.

Important

factors

which must be considered at the

conceptual stage

of he

design process

are

the

questions

of

workshop

acilities

-

including

size

-

and

transportation

between

workshop

and site. Whilst

long girders

or

large

sections

may

appear

to be

desirable,

in order to reduce

the numberofsite

connections,

thiscan

reduce the numberoffabricatorswho could tender

for

a

given project.

In the United

Kingdom,

road

transport

is

normally

used

and loads

up

to 2.9 m

width,

18.3 m

long

and

76,200 kg weight

may

be

moved

without

any problems.

Above

these dimensions he

Police need to be notified of "Abnormal IndivisibleLoads"

and

indemnity

to

Highway

and

Bridge

Authorities is

required.

Where the dimensions exceed width 6.1

m,

length

27.4

m,

or

weight 152,400 kg

a

Department

of

TransportSpecial

Order is

required.

(Reference

'Abnormal Indivisible

Loads',

"Aide Memoire

forRequirements

as

to Notice and

Authorisation

whennot

complying

with Construction

and

Use

Regulations",

Source:

Director

(Transport),

Departments

of

he Environment and

Transport).

It should be noted

that the various

police

authorities have different

periods

when abnormal

loads are allowed

to move

through

their districts. If

neighbouring

"times" are

significantly

out

of

phase

and

general traffichold-upscause

disruption

to

the

movement

of

abnormal loads

it is

possible

for the latter to be

delayed by

up

to 24 hours. Ifone or more

cranesand

associated erection

staff are held

up by

these enforced

delays,

the additional costs can be

very

significant.

Certain towns and cities

place length

restrictionson materials which can

be moved

by

road

e.g.

certain areas of London restrict

lengths

to 12 m.

Girders canbe fabricatedand

despatched lying

flat,

the overall

height

of he load is

dependent

upon

the route travelledand the clear

height

of

any bridges likely

to he encountered. Rail

transport

can accommodate

long pieces,

butwidth and

height

are

more restricted.

One solution o limit the

length

and

height

ofunits

being transported

is to use a

system

as

illustrated in

Figure

1. The two external sections are

shop

welded

and the central section is

siteor

shopassembled;

the whole

being

bolted

together

on site. The

completed

rafter can be

craned into

position.

For

export

where

shipment

s

involved,

pieces up

to the samedimensions

as for road

transport

may

be accommodated but it should be

appreciated

that

shipping

charges

are often based on

volume rather than

weight.

Often there are

relatively

severe

restrictionson the

length

of a

piece

that canbe carried in the hold ofa

ship.

The

ship'sengineer may

refuseto

carry

the

steelwork as deck

cargo.

It

may

be found moreeconomical to

despatch

the steel

piece-small

for

subsequent

assembly

on site. Care must then

be

taken

to

ensure that

the site work is

satisfactory.

Other factors of

importance

which can influence the economics of his

type

ofconstruction

are

the facilities available for fabrication and for erection on site.

2


12/100

Many

fabrication

shops

now have

equipment

which can cut and hole steelworkin

a

semi-automatic manner

thus

reducing

direct labour costs.

Jigs

can also be used for the

rapid

assembly

of

components.

All these tend to make latticeconstruction more attractive.

On site

the

lighter

overall

weight

of

ndividual

components

can result in the

use

of

simplelifting

equipment;

site costsrise

appreciably

if

heavy

cranes have to be installed for erection

purposes.

For the

design example

in this

publication

it is assumed thatthe

building

is for the home

market and that

a

well

equipped

fabricator will manufactureand erect the steelwork. It

follows that the

design

must be in accordance with the

appropriate

British

Standards,

codes

and

regulations.

Brief

explanatory

notes on

these

publications

are

given

in

Section 3.

External

Central

section section

7,7,7

Z7rr

Figure

1

Sectioned

girder

3


13/100

3.

STANDARDS

AND CODES OF PRACTICE

3.1 British Standard

5950

-

Structural

use of

steelwork in

building

This

document' is in nine

parts combining

codes

of

practice

to cover the

design,

construction

and

fire

protection

of

steel

structuresand

specifications

for

materials,workmanship

and

erection.

The relevant

parts incorporated

into this

publication

are Parts 1 and 5.

3.1.1 BS 5950: Part 1: 1990 Codeof

practice

for

design

in

simple

and

continuous construction:

hot

rolled sections

This limit state

specification provides limiting

valuesfor

strength

and deformation or various

elements

which

form

part

of

structures,

and for whole

systems.

The document' covers

aspects

related

to hot

rolled

sections

i.e. UBs, UCs, angles,

channels,

hollowsections,

etc.

3.1.2 BS 5950: Part 5: 1987 Code of

practice

for

design

ofcold formed

sections

This

specification2, using

limit state

philosophy, provides limiting

valuesfor

strength

and

deformation and identifies

full

design procedures

and

empirical

methods.

Within

this

publication,

it

is used in the

design

of

purlins

and

side

sheeting

ails.

3.2 BS

5502

-

Buildings

and structures for

agriculture

Various

parts

which cover

materials,

design,

construction and

loadings3.

3.3

BS

6399:

Part

1: 1984

-

Design Loading

for

Buildings

This is

a

"Code

of

practice

for dead

and

imposed

loads"

for

use

in

designing buildings(4

(this

is

provided

as

a

revisionto

CP3

Chapter

V

Part 1: 1967 which it

supercedes).

3.4

BS

6399:

Part 3:

1988

-

Code

of

practice

for

imposed

roof

loads

This is a "Code of

practice

for

imposed

roof oads" and in

particularsuggests

methods

of

considering

snow loads for various

buildings5.

The loads can be usedfor

permissible

stress

design

or where

factored loads

are

adopted.

This code

recognises

the variation

in

snow

loading throughout

the United

Kingdom

and the

effectofvariable

snow

loads on

a

roof due to

drifting

effects.

3.5

CP3: Chapter

V:

Part

2: 1972

- Wind

Loads

The

effectofwindon a

building

has been foundto be

very complex

and

dependentupon

many

factors such as the

geographical location,

the

shape

of

he

building

and its

relationship,

to other

buildings

and natural

features.

The

various rules for

calculating

the

design

wind

loadson a

structureand its

cladding

are

given

in this codeof

practice6,

supplemented

by

a

guide published by

the

Building

Research

Establishment7.

4


14/100

This code will be

replaced

by

BS 6399: Part 2.

3.6

Statutory

regulations

In

addition o the above the

buildings

must

comply

with the

requirements

of

he

Building

Regulations,

which

apply

in

England

and

Wales,

and where

appropriate

with the

special

variationsor

equivalent egulations

applicable throughout

the UK. Particular thermaland

sound insulation

equirements

of the

cladding

must also be met. For

buildings

outside the

United

Kingdom

the local

regulations

must be observed. Whilst

many places accept

structures

designed

to BritishStandards care must be taken to consider

any

unusual features such as

typhoons

or

earthquakes.

3.7

National structural

steelwork

specification

for

building

construction (2nd

Edition)

The

object

of

this

publication8

by

BCSA and SCI is to achieve

greater uniformity

in contract

specifications

issued with tender and contractdocuments.

3.8

Quality

assurance

BSI

Handbook

provides

a

comprehensive

document

of

he relevant standardsassociated

with this

topic.

Of

particular

interestto the

designer/fabricator/erector

is BS 5750 :

1987'°

which

provides

a

three level

specification

of

QA

requirements

in

the contractual situation.

5


15/100

4.

BUILDING

FORM

4.1 General

Before

proceeding

o the detailed

design

of

a latticeframed roofit is desirable to consider the

alternatives available.

At the outset

is

must

be

appreciated

that ifan industrial

building

is

to be

warm

during

the

winter

and cool

during

the summer some form of

heating

and ventilation s

required

in

addition to the thermal insulation called for

by

theThermal Insulation

(Industrial Buildings)

Regulations.

The roof

space,

which willbe heated with the rest

of

he

building

unless cutoff

completely

by

a

horizontal

ceiling,

is a constant

charge

on

running

costs without

contributing

to the work

space.

There

are, therefore,

financial

advantages

in

keeping

the

roof

space

to

a

minimum

bearing

in mind that services can be accommodated in this

space.

This can be

achieved

by keeping

the roof

space

as shallow as

possible,

commensurate with

economy

of

initial

cost and

efficiency

of

he

cladding.

A

flat

roof,

or a roof

with

only

a

nominal

camber,

can reduce the

roof

space

to

the minimum

but

may

be

expensive

to

build since the

roof

cladding

will have

to be

of

a

more

sophisticated

natureto ensure

adequate

weather

protection. Again,

with

a

flat roofof

any

reasonable

span,

deflection

of the

structure

or

girders

becomes

important

and extra steelwork

may

be

required

merely

to reduce it. A

portal

frame

design helps

to reducethe deflection but it does not

reducethe costof he

cladding

and the

provision

of

he

necessary

rigid joints

is an

added cost

on the

structure.

Probablythe most

economical

form

of roof

construction

is

one of

low pitch (say50

which is

the

preferred

minimum)

on

which a

simple

form of

cladding

canbe used with success and

whichat the same time reduces

deflection

whilst

maintaining

reasonable

heating

costs.

However care is

required

in the

selection

of

he

type

of

sheet,

the

type

of

fixing

and the

sealing

of

end

laps

(which

shouldbe

avoided,

if

possible). Special

care is

required

where

translucent

sheets

are

required

(see

section 5.3 on

cladding).

For other than raised seam

roofing

7½

°

is the

preferred

minimum

slope.

4.2

Low

pitch

roofs

Such low

pitch

roofs can be

supportedby

either solid web

beams,

castellated beams or lattice

frames.

Each has

advantages

and

disadvantages

which

must

he

examined

beforea decision

can

be

made.

4.2.1

Solid web

beam

This

is the heaviest form

though relatively simple

and

cheap

to make.

However,

the

depth

of

section

satisfactory

for

structural

purposes may

be

too shallow

for

the

penetration

of

service

ducting.

A monorail or

underslung

crane can be

supported

at

any position

but local

stiffening

of the section

may

then be

required.

4.2.2 Castellated beam

This

is a

method of

increasing

the

sectional

properties

of

a beam without

materially

increasing

the

weight.

The

roof

space

increases

but

some services can

be

accommodated in the

castellations. Monorails canbe locatedas

required

but it

may

be

necessary

to fill in local

castellations and stiffen the

flange

to

carry

the

load. Castellated beams increase

the

bending

strength

and flexural stiffness

quitesignificantly.

Enhanced shear

capacity

at

points

of

high

shear canbe accommodated

by

filling

the castellations in that

region.

6


16/100

4.2.3 Lattice

frame

Figure

2 showsthree

different

types

ofrafter and

indicates

thefacilitiesfor services

and

monorails. It

also illustrates

hat,

notwithstanding

its extra

depth,

the lattice frame

has a

distinct

advantage

where services

have to be carried in the

roof.

In

addition,

the

reduction

in

weight

of the

girder

can result in

economy

in

the

supporting

structure and

foundations.

This isthe

lightest

form

of

construction

though

it

requires

more

fabrication.

The

roof

space

increases

butservices can

usually

be

accommodated

withinthe

depth

of

he

girder.

Monorails

supported

at the

panel points

cause

little

problem,

but

if

they

are located between hem

some

local

stiffening

may

be

required.

The latticed

girder

will have

a much

larger

second moment

of areaand section

modulus

(about

XX

axis)

than a

corresponding

solid

web beamofa similar

weight.

Therefore there will be

enhanced

strength

and

stiffness.

Type

(c)

Figure

2

Typical

roof

girders

Monorail

7

Solid

web

beam

Type

(a)

Type

(b)


17/100

5. LATTICE

FRAMED ROOFS

5.1

Simple

forms

Depending

on

the overall

dimensions of

he

building,

he latticeframed

roof

can

take

many

forms, some

of

which are

examined below:

5.1.1

Single bay

low

pitch

roof

Economically

spans up

to 30m

are

often fabricated

using

standard

UB,

UC

section

portals.

Above this

span lighter

rafters are

provided by

latticed

girders,

as shown in

Figures

1

and 3.

The

advantage

of

he

horizontal

boom is that

designing

for

the "kick out"

effect,

Figure

4,

is

removed. Columns

are then

only

designed

for

axial

load and

moment

(due

to

the

eccentricity

of

he

load)

from the

roof,

in

addition to

wind load

on

the

vertical

cladding.

A

factor

to be

considered s

the

possible

lengthening

of

he

bottom

boom

due to

tensile

strain.

5.1.2

Multi-bay

low

pitch

roof

Eaves

displacement

The

single

low

pitch

roofcanbe

extended

into

a

series

of

similar

bays (Figure

5).

Alternate

stanchions

in the

valley

can

be

omitted,

the

intermediate roofframes

being

carried on

a

longitudinal

valley girder,

spanning

two

longitudinal

bays,

as

indicated.

5.1.3

Single bay monopitch

roof

When the

slope

of the

roof is

low it

is

sometimes

advantageous

to

use

a

monopitch

roof

(Figure

6).

The

extra roof

space

can

be

compensated

for

by

the

saving

in

drainage

since

a

gutter

is

requiredonly

along

one

edge

and not

wo.

Monopitch

roofs are

mainly

used

for

relatively

small

spans.

8

Figure

3

Single

bay

ow

pitch roof

—

—

—

/

Figure

4

"Kick out "effect


18/100

Figure

6

Single

bay

monopitch

roof

5.1.4

Multi-bay

roof

In

combining

rames

to

obtaina

multi-bay system

alternatestanchions can

be

omitted

(Figure

7).

The

roof

is

supported

at

the

apex

and the

valley by girders spanning

two

longitudinal bays. Alternatively

a

multi-bay

frame can

be

provided using

a

multi-monopitch

roof

arrangement(Figure

8.)

It

is

preferable

o

ensure that a

valley gutter

is wide

enough

for

an erector

or

maintenance

operative

to

stand in.

In

the

alternative case

using

mono

pitch

roofs

(Figure

8)

the lattice rames

all

slope

in

the

samedirection. Extra

gutters

are

required

but

advantage

can

be

taken

to

introduce

lights

above the

valley gutters.

This

system

is

particularly

useful

if

direct

sunlight

intoa

building

is

to

be

avoided. The

glazing

can then

be

provided

in the north

facing slope

of

the saw-toothed

roof.

Eaves

gutter

Ridge

Cladding

Side

cladding

Figure

5

Multi-bay

pitch

roof

flashing

Side

claddi

bolts

Side

Ridge

flashing

Longitudinal

girders

Stanchionsat alternate

frames

Figure

7

Alternative

multi-bay

pitch

roof

9


19/100

5.2

More complex forms

North

light

Cladding

Where

large

internal areas are

to be

relatively

free

of

stanchions,

a

double

latticed

system

can

be

adopted.

Here,

secondary

frames

in one

direction

are

supportedby

primary

frames

spanning

in

the other direction

between

widely spaced

stanchions.

These notes

on

lattice

framed

construction

would

not

be

complete

withoutsome

reference to

more

complicated

forms built

up

of

attice

frames or

lattice

girders

and

trusses and

of

space

frames.

5.2.1

Umbrella

roof

In this

orm

of construction

light

trusses are

slung

either side

of

main lattice

girders

(Figure

9).

The

pitch

of

he

roof

must be sufficient to accommodate

the

main

girders

which

in turn should be ofsufficient

depth

to avoid

excessive

flexibility,

bearing

in mind

the

incidental

application

of

imposed

and wind

loading.

Care

needs

to be

taken

to

ensure

adequate

provision

for

drainage

of rainwater.

The

trusses

act as

cantilevers

with the

bottom

chord

in

compression

from

imposed

loading

but

wind

loading may

cause

a reversal ofstress.

Since

these

compression

members are

not

laterally

restrained

(in

normal

truss

construction

the

rafters

are the main

compression

members

and

they

are restrained

by

the

purlins

etc.)

a

system

of inclined or horizontal

bracing

is

required.

Eaves

Ridge

cladding

Figure

9 Umbrella

roof

10

Figure

8

Alternative form

of

multi-bay using

monopitch

roof

Roof

—

Stanchion

—

Cantilever trusses

Floor,

level


20/100

5.2.2

Space

frames

When

large

areas need to be covered

by

a

roof,

with

minimum

use

of nternal

columns,

a

possible

solution is

to

use

a

space

frame.

Generally

hese are formed

of

tetrahedrons

as

shown

in

Figure

10. In

principle, parallel

series

of

lattice booms

(top

and

bottom)

are

connected

by

a

system

of

diagonal

members to form a latticed

2-way

spanning

plate

of

significant stiffness.

Angle

section

upper

ch

bars

11

Tubular

Secondary

tie

bars

Space

deck

module

Figure

10

Typicalspace

frame


21/100

5.2.3

Butterfly

roof

The

butterfly

oof

(Figure

11)

is

unlikely

to

have the

drainage problem

of

he umbrella roof.

Since the lattice

girders

do not

directly

govern

the

slope,

the roof

can be flatter. The lattice

girders being placed

in the

valleys

do,

however,

call for

increased roof

space.

5.2.4

General

comment

These

various

forms,

and indeed

many

others,

are

frequentlyadopted

to suit

the

requirements

ofa

particularproject,

but

it

must be remembered that

they

can increase

he unit

cost of

a

structure

compared

with the more

simple

forms.

Side

cladding

Figure

11

Butterfly

roof

5.3

Cladding

Cladding

to

a

building (roof

and

walls)

has to be

provided

to

satisfy

aesthetic

and functional

criteria and to

satisfy

the economics

of he

project.

A

satisfactoryappearance

is

accomplished by

selecting

the

appropriate

colour

and

shape

to

blend in with the remainderof

the

building

and

neighbouring

structures.

A useful "ProductSelector" for

"Roofing

and

Cladding

in Steel" has been

produced by

BSC

Strip

Mills Products' . This

provides

details

of

about 70 different

products.

Functionally,

he

system

has

to

provide

resistance o

atmospheric

conditions,

sound

transmission,

and

light

reflection.

It is essential to ensure that both roof

and walls are

watertight

under all

conditions,

wind causes no

damage

to either

cladding

or

structure,

and

adequate

insulation is

provided against

heat and

cold.

Structurally,cladding

has to be

of

adequate

strength

and stiffness o resist induced stresses

and excessive deformation. Profiled

sheeting

s

commonly

used since it satisfies

these

requirements

and is

additionally light,

durable and

easy

to erect

quickly.

Coated steel sheets are

extensively

used

for

cladding

all

types

of industrial

buildings. They

are available in

a

wide

range

of

profiles(rib depths)

and colours.

Many proprietary

cladding

productsprovide integral

insulation

systems,

making

use

of

expanded polystyrene

or

similar

insulationmaterial. Doubleskin metal

systems

are available and are considered

by

some

designers

to be the best

type

of

cladding. Clearly

where

composite cladding systems

are used

there

is

only

one

operation

for the erectors.

12

Roof

Eaves

ci

H.D.

bolts


22/100

In

general

a

single

skin is usedfor

stores where heat retention is nota

significant

factor

e.g.

timber stores

etc.

In

factories and offices where the

envelope

is

dependent

on the

"U'

value,

double

skin

cladding

is a sensible

solution.

However, lining

sheets

may

be a

critical

factor in the

design

for windsuction.

Sheets,

supported

by purlins (Figure

12),

are

available

in

long lengths. Where

possible,

sheets

are

lifted

into

position

by

cranes to

provide

better

safety

conditions for the fixer.

Hencethe numberof

aps

should be minimised in order

to

reducethe

possibility

of

water

ingress,

particularly

on shallow

slopes.

It

is

possible

o

vary

the

spacing

of

supports

for

cladding depending

upon

the thickness and

shape

of he

profile.

Three factors

generally

control the

spacing.

The first is

purlin

size and the second is the limitations of

lining

supports.

Often

the

length

of the inverted

'T'

sections usedto

support liningpanels

is limited

to about 1.8

m,

consequently

purlin

centres are restricted

to that

dimension.

Finally purlins

are often used to

provide

lateral restraint o the raftersor frames. Allof hese factors need to

be considered

to determine he most economical solution to the

roofing

system.

Aluminium

sheeting

s similarto steel

sheeting,

although

it tends to be

lighter.

The

aluminium

coating

may provide

better resistance

o industrial

atmospheres, greater

solar heat

reflection and

brighterappearance.

Natural

lighting

can be

provided

by

the introduction

of

ranslucent sheets

(which

structurally

can

be

very

weak),

or

stretches

of

patent glazing.

The latter is

clearly

more

expensive

and is

often

limited

to

slopes greater

than 12°.

Translucentsheets canbe moulded to the

profile

of

the main

cladding

and would use similar

fixings.

Care must be taken in

positioning

oof

lights.

It is

generally

necessary

to have

a

metal

or

similar main

cladding

sheet

at

the

top

and bottom

of the roof

light

in order to

provide

adequate strength

to the

system.

When

lights

are

placed

near to the eaves and/or

ridge

there

may

be

inadequate support.

Cladding

can be

fixed

by

the use

of

self

apping

screws or hook bolts. Self

apping

screws

may

have recommended

torques.

An

aspect

to

be

carefully

considered is the thickness

of

the

purlin.

It is essential to ensure there

is

sufficient thickness

of

metal

to

accommodate

self

tapping

screws. If here is

any

doubt it is

advisable

to

check

with the

cladding

and

purlin

manufacturers of the

adequacy

and

safety

of

he

composite system.

Screw sizes vary

and

their

strengths

are

dependenton their

"pull-out" capacity.

In

checking

these the screw

manufacturer has to take intoaccount the

high

"local" wind suctioneffects.

Often

gutters

are

placed

inside

at

eaves level

to

provide

enhanced

appearance.

However,

his

advantage

needsto be

weighed against

he

difficulties

which

may

be encountered in the

repair

and maintenance of he

gutter.

With this

system

the use

of

overflow weirs should

be

considered

to

allow

for

blocked

pipes

and freakstorms.

5.4 Purlins

Purlinsare

required

to

support any

of he

types

of

cladding

available.

Cold formed

sections

have been

developed

to

provide

elements of

adequate strength

and stiffness which also allow

maximum

speed

of

erection.

If

the

design

criteria

is

such

that cold formed sections are

inappropriate

hen use can be made

ofhot

rolledsections.

13


23/100

For frame

spacings

between6.0 m and 10.0m

a

propped purlin system

can

be

adopted

constructed from either

light

angle,

tee or channel sections

or

structural hollow

sections,

as

shownin

Figure

12.

For even wider frame

spacing

he

use

of

attice

purlins

should be

considered.

They

canbe made

up

in

many ways, e.g.

using

flats with rod

lacing

or small

structural hollowsections.

(Cold

formed lattice

purlins

are also

available).

Castellated beams

have been used on occasions.

It shouldbe noted thatboth

propped

and lattice

purlins

can beuseful for

providing

estraintto

the bottom

of

he main

supporting

rames.

As

indicated

in

Section 5.3 on

cladding

it

may

be

necessary

to limit the

purlin

centres to

1.8 m

(generally

fabricators

prefer

1.7

m to

1.9

m).

Of

particular

consideration is the locationof the

purlins

relativeto the node

positions

of the

lattice frame.

If

they

coincide with the nodes then the

top

boom would

only

transmitaxial

loads.

If

they

are located betweennodes then

bending

is induced in the boom member

in

addition

to

axial

forces.

The

span

of

purlins may

be controlled

by

a

fixed

specification

for the main frame centres.

Alternatively

frame centres

can be determined

by

selecting specific purlins

which

may

have

limiting spans.

Cold formed

sections are

normally

available in

lengths

up

to 10 m and

depths

from 120 mm to 300 mm.

Normallyspans

are of the order

of

4.5

-

6 m. To enhance he

lateral stiffnessof the

purlins

it

is

sometimes

necessary

to use

anti-sag

bars

-

Figure

16.

This,

however,

can increase labour

costs and therefore their use should be

weighed against arger

purlins

orcloser frame

centres.

An

aspect

to

be considered concerns the

design

for snow loads. Cold formed

purlins

have

generally

been

developed

on the basisof tests carriedout

using

uniformly

distributed oads.

Snow

loading may

be

trapezoidal

and care is

required

in the

interpretation

of

he

manufacturers' iterature.

A

further

design

criteria which has

implications

on

purlin

size is the

incorporation

ofa

dominant

opening

in

the

sideofa

building.

This can

significantly

increase he

uplift

due to

wind.

Purlins are often

usedto

provide

ateral restraint

to

the

compression flange

of the

main

supporting rames, and

to

transmit wind

loads

to the

bracing

system.

If

his

is the

case

combined

loading

needs

to be

considered when

selecting

the

appropriatepurlin

i.e.

it

could be

subjected

to

the maximum dead

plus superimposed (snow) loads,

which induce

bending,

and

additionally

axial

load from windeffects.

Eaves

purlins

are

also available which have a

sloping op flange.

Various

types

of

purlins

are

shown in

Figure

12.

5.5 Side

rails

In

general

the

comments

provided

in

the

previous

Section

on

purlins

are

applicable

to

side

rails. The loads

acting

on these will be different since

vertical forces are induced

by

the self

weight

of he

cladding

which acts

perpendicular

o

the wind loads.

Sheeting

rails are often

fixed at about 1.8 m.

Generally,

a

limit

of2

m is

placed

on their centres.

Anti-sag

rods

are

more

easily

fixed to stiffen

theseelements.

14


24/100

Asbestos

cement

sheets

Self

tapping

screws

Steel

sheets

i

5°)L

S%iRaft:):;;:ul:tion

Cold formed Z

(Anti-sag

bars

required

for

spans

over

4.5

m)

'Structural hollow section

(circularor

rectangular)

Propped angle purlin

Sheeting

and insulation

Lattice

purlin

—

Roof

girder

Figure

12

Types

of

purl/n

15

Purlin

Lattice

girder

Purlin

stays

Hook bolts

Hook

Sheeting

Rafter

Angle

Insulation

Rafter

Roof

girder

Props

to bottomof roof

girder


25/100

6. CONCEPTUAL DESIGN

Before consideration s

given

to the method

of

analysis

and

design

o be

adopted

certain

decisions

have to be

taken,

which

may

laterbe

modified

as the

design

progresses.

The

effect

of

any

modifications

clearly

can alter the detailed

design

and alterations to calculations would

ensue.

Thereare

four

principalcomponents

ofa

light

industrial

building

i.e. the

cladding,

the

cladding supports,

the

main frame and the foundations.

Early

decisions are

required

on

type(s)

of

cladding

and

type

of

purlin

and

sheeting

rails.

Since these are all

supported by

the mainframe.

If he frame

is

considered

as a

simple portal, Figure

13,

it is

necessary

to decide on the

type(s)

of

fixity

to

be

provided

at the

base,

eaves

and

ridge. Generally,

the columns

to

the

frame will

be of

I

or H

section,

unless the

building incorporates

a

high capacity overhead

travelling

crane when a

composite

column

might

be

required.

If he rafters are to be latticed structural steelwork it is

possible

o use different

layouts

of the

internal

members, Figure

14.

However,

since the

diagonals

are

likely

to be

subject

to

stress

reversal,

due to wind

effect,

the warren

type

truss is

generally preferred.

In

selecting

he

layout

it

is

necessary

to decide on the

position

of

purlins.

If

hese are located

at

node

points

then local

bending

in individual

top

boom members are avoided. In

principle,

forces in all of

the members are either direct tensileor

compressive,

with

bending

and shear effects

being

secondary,

as

a

resultof

deformation

of the truss.

Analysis

of

he

framework

can

be

carried out

by

hand

calculation,drawing

or

computer.

In

the firsttwo

methods,

it is essential to assume that all

joints

are

pinned

and

preferably

end

support

conditions to the rafters are such thatthe truss is

statically

determinate.

Whena software

package

is used there are a number of

options,

three

of

hese are:

(i)

assume all

joints

of he truss and the connections to the columns are

pinned;

(ii)

assume full

rigidity

of all

joints;

(iii)

assume the internalbracing

members

are

pinned

to

the boomswhich are

considered

to

be

continuous

and

therefore

rigid.

In

adopting

(i)

or

(iii)

it is

necessary

to

consider

the

possible

effectof

secondary

stresses

caused

by:

(a)

loads

applied

between he truss

nodes;

(b)

moments

resulting

from the actual

rigid joints

and trussdeflections.

Additionally,

in all

cases

care needs

to be

taken

in

member

layout,since

secondary stresses

can be induced

by eccentricity

at the connections.

(Specific

reference should be made to

BS 5950: Part

1,

Clause4.l0'and Structural Steel

Design'2 by Dowling,

Knowles and

Owens),

Dowling

et al

suggestsecondary

stresses should be calculated for

heavy

trusses used

in industrial

buildings (e.g.

those

supporting

overhead

cranes)

and

bridges.

It

is

traditionally

recognised (e.g.

in BritishSteel

Publication,

Designof

SHSWeldedJoints'

))

and

Dowl

ing

et

al also

suggest

that latticed structures are

assumed,

for

design

purposes,

to have

pinned

joints.

This

may

leadto

higher

defiections

than those

induced

in a

rigid jointed truss,

but in

practice

16


26/100


27/100

An

example

of

composite

form is shown

in

Figure

15 where the booms are of UC section and

the internalmembers RHS. The UCs enable

easy

connectionofservicesto the truss and

easy

connection to columns. Also

bracing

in

the

plane

of

he roofcan be

provided using simple

in

plane

membersand

simple

connections,

or

by using

the relative stiffnessofan I

or

H

section.

When hollow sections

are

used with welded oints reference

should

be

made

to

the British

Steel

Publications,

listedin Section 7.5.

It

is essential to ensure that

it

is

possible

to make a

fullweld. Difficulties can arise

where

large

booms and small internal members are used

which

may

requirejoint

stiffeners. These

may

be

expensive

and it is

likely

to

be

prudent

to

increase he member size. The

designer

must be aware

of

problems

which can arise in the

detail

design

at the

joints.

The

specific advantages

ofhollow sections

(and tubes)

when

compared

with traditional

sections

(UBs,

UCs, Channels,

Angles

etc.)

are the

high strength

to

weight

ratio,

maximum

efficiency

in

tension, efficiency

as

struts,

good

torsional

properties,appearance

and

maintenance. In

deciding

to use CHS or RHS the

designer

should remember that some

fabricators

are

not fully

equipped

to

use circular hollow

section.

Their main

disadvantages

can be the

higher

cost

of

connections

especially

at nodes

involving

overlapped

CHS

bracings

and

chords,

the relativedifficulties of

making

on site connections

for services

(electrical etc.)

and

higher

basic costs than traditional

sections on

a

tonnage

basis

(overall,

however,

lighterweight

frames are

produced).

Relevantto the

design

code BS 5950: Part 1(1) is the consideration

of

sectionclassification

(Fable

7

of

the

code).

Tees cut from UBs are

generally slender,

hencea reduced

yield

stress

has to be used.

Tees cut from UCs are not affected in the samemanner.

In

designing

the

joint

it

is

necessary

to examine whether

high

local stresses willbe induced

by

the selected

arrangement

and member sizes. These

high

local stresses

may

even occurwhen

member axes intersect.

The relative

slopes

of he internal members are relevant

to

the

detailing

for

the fabrication

process.

If

hey

are

parallel

to each other then the

angle

ofcut

at

each

end is identical for all

members.

The final decision

on the

type(s)

of

member(s)

to be used

may

be influenced

by

aesthetics and

not cost.

CHS

UC UC

RHS

OHS

RHS RHS RHS

CHS UC UC RHS

Figure

15 Alternative lattice

girder

layouts

18


28/100

7.

PRINCIPLES

OF

DESIGN

The

design

f all the steelworkfor low rise latticeframed

buildings

should

satisfy

the "aims

of

economical

structural

design"

and "limit state"

philosophies

outlined

in

the

appropriate

Codes

of

Practice.

Basic

designassumptions

are madeas to the

behaviourof he various units which make

up

the

structure.

7.1

Purlins and side rails

Purlinsand side

rails can be

designed

to

satisfy

the

strength

and deformation

requirements

of

the

appropriate

codesor

they

canbe

designed usingempirical

rules

given

in

Clause 4.12 of

BS 5950: Part

1'

and Section 9 ofBS

5950: Part 5(2)

It

is

of

note that the

empirical

rules are based on unfactored loads and also that the tablesof

section

properties(A

checklist

for

designers'6 published by

the

SCI)

do not list

plastic

moduli

for

angles.

Purlins are

generallydesigned

as

continuous

members,

over two or more

spans,

supporting

uniformly

distributed

loads.

In

this case connections have to be madeto transmit

shearand

bending.

Cold formed

sections canbe selected from manufacturers'

catalogues

where

it

is

guaranteed

that the

carrying

capacity

of

the various

systems

is

based

on

the results

of

extensive research

and

development.

Continuity

is

obtained

by

the use of

sleeves,

and the effective

ength

of

purlins

are reduced

by

the useof

anti-sag

bars

(Figure

16).

When

applied

loads

are not

uniformly

distributed

e.g. trapezoidal

snow

loading

or when

purlins

are used

to

support

ventilation

systems

etc. then

original

calculations are

required.

These will

make use ofBS 5950: Part 5 and section

properties

for cold

formed

purlins

provided

in

manufacturers'

catalogues.

7.2

Lattice framed

roof

girders

As indicated n Section

6

the

design

will

be

based

on

the

assumption

that

joints

are

pinned,

rigid

or

a

combination

of

he two.

The

girder

will

support

vertically applied

dead

and

superimposed

loads

plus

wind

loads.

The

latter is

likely

to

induce

stress

reversal

in the

members. The rafter will also transmit the

horizontal wind loads from the

vertical

cladding

and

may

act to transmit wind loads in the

plane

of the

roof.

Typicalload directions are shown

in

Figure

17.

7.3

Stanchions

When

pinned

bases are

adopted

then moment

fixity

is

required

at the column head. The

column willbe

designed

for

axial and shear forces

only

at

the bottom but

for

axial,

shear

and

bending

in

the

upper length.

Use offixed bases enables the stanchions to be

designed

as

propped

cantilevers,

although

it

should

be

noted

that

simply

linking

the

top

of he

stanchions

with

the roof russes does not

provide

a

fully rigidpropped system.

The column heads and

19


29/100

girders

can all move

together.

It

is

of

note that the relative

stiffness

of

he rafter and column

are

significantly

different

(possibly

of

he order

of

4 to

1).

Also

changes

in

the overall

depth

of he

rafter can

significantly

increase

or

decrease

the stiffness

of

hat member.

The stanchion

size

is

controlled

by

its effective

ength,

which is

likely

to

differabout

orthogonal

axes. Care is

required

in the selection

of

end and intermediate

fixity

conditions.

Reversible

wind

oads

Figure

17 Frame loads

20

rail

Cleat (behind)

rafter

Figure

16 Sleeved

purlln system

I

I

\

Dead &

I

Vertical

imposed

loads

Reversible

wind loads

I

Vertical

cladding

(dead)

load


30/100

7.4

Bracing

Bracing

must be

provided

to

accommodate

wind

loads

on the

gable

columns. This can be

usedto facilitate

plumbing

and

squaring

the

building

during

erection.

It can also

provide

essential

stability

o

the steelwork

during

erection.

Bracing

normally

consists

of

diagonal

members betweencolumns and

trusses both

in the

walls

and

plane

of he roof. The

bracing

canbe

singlediagonal

or cross

members

(Figure 18).

If

the former

system

is

adopted

the members are

designed

o

supportcompressive

and tensile

loads. When cross

members are used

only

the members

in

tension

are assumed to be

effective,

those in

compression

are

designed

to

satisfy

the

slenderness

criteria,

Clause 4.7.3.2

of

BS 5950: Part 1: 199O'.

When

masonry

is used

as all or

part

of he vertical

cladding,

it

is feasible

to

use

that

element

as

part

of

the

bracingsystem.

/\NN/7NNNN

Single

diagonal

roof

bracing

><

><

x

<

Cross

member

roof

bracing

Figure

18 Roof

bracing

7.5

Connections

A

very

important

aspect

of

design

using any

material is the

design

of

connections. Structural

membersare

designed

to

carry

axial

loads,

shear

force, bending

moment and

torsion.

Consequently

connections

must be

designed

to transmit these

forces from one element to

another without

inducing

excessivestresses

or

deformations.

To

produce

a

good design

ofa

complete

structural

assembly

it

is essential for the

designer

to

clearly

state

at

an early

stage

the basic

methods

by which various

members

are

to be

joined.

Sophisticated

methods of

analysis

are nowavailable

to

determine

o a

good degree

of

accuracy

theforces and deformations

throughout

both

simple

and

complex

structures.

This

degree

of

sophistication

is

not however

generally

available

in

connection

design.

The stresses induced

by

connections are often

indeterminate and their distribution

throughout

a

joint

is

not

always

consistent even

in

identical conditions. Stress is

always

a

function

of

deformation and the

latter

can

vary

with the

irregularities

of the

properties

of he

members

being

connected,

the

type

of

fasteners,

the

quality

of

workmanship

in

making

the

connection

and "built

in"

stresses

in

the

parent

members.


31/100

Most

connection

design

is,

at

present, only approximate.

The essential aim is to

provide

the

type

ofconnection

stipulated by

the

designer

which is

efficient,

economical and

aesthetically

pleasing.

The latter is not

always

essential. Use

will be made

principally

of

he basic lawsof

statics i.e.:

EX

=EY =EZ

=0

=

EM

=

EM

=

0

i.e. all

joint

behaviourwill be considered to be

statically

determinate. The distributionof

internal forces in a connection has to be assumed and either elastic

or

limit state

design

may

be

appropriate.

The fabricationof

connections

is

particularly

abour intensive and thereforein order to

keep

overall

costs down

it

is

necessary

to

try

to

produce simple

but efficientmethods

of

oining

members,

by welding

or

bolting.

In

general

the

design

of

connections

will follow the

recommendations

given

in

BS

5950:

Part 1:

1990,

Section Six. Connections.

In the

case

of

he

following designexample using

hollowsection the

design

is

carried out

using

as references he

following

publications produced

by

British Steel viz:

Design

of SHSWelded Joints TD338'

Jointing

TD

325

Welding

ID

328'

Hot finished

structural

hollow

sections;

sizes, propertiesand

technical

data TD

167

Useful

reading

in the first instance is TD 325 which

provides

an indication of he wide

spectrum

of

application

ofRHS.

Publication

TD

338

provides

a

clear method

of

Designing

SHSWelded Joints. As indicated

in

Section Six

of

BS 5950:

Part

1,

it is

common

practice

to

carry

out the

analysis

on the basis

of

pin-jointed

frames with members in direct

compression

or tensionand the centre lines of

members

intersecting

at the

nodes,

as shown in

Figure

19. Often it is

necessary

to

provide

a

gap

or

overlap

as shownin

Figure

20. Joints

may

take

a

variety

of

geometric

forms as

shown

in

Figure 21.

TD

338 detailsthe

method

of

establishing

thejoint's

design capacityin

limit

state

terms, compatible

with

BS

5950 and Eurocode3.

It

should

be

noted that fillet

welds

generally provide

the most economic method of

connecting

members in

structures

subject

to static load.

Clearly

one

exception

is the case ofend to end

connections where butt welds canbe

provided

to

develop

the full

strength

of the

sections

connected. In this

case with RHS sections internal

backing

members are

provided,

which are

formed from

strips

20-25 mm

wide and 3-6

mm

thick.

Of

note is the recommendation ina

paper by

N

Yeomans,

New

Developments

in the use

of

StructuralHollowSections17:

"Because of he influence of member and

joint

geometry

on the

jointbehaviour,

it

is

important

that

engineers design

the

joints

when

determining

member

sizes;

with

SHS

design

this

job

should not be left to the detailer".

22


32/100

Figure

19

Noding oints

a)

Gap joint

with

positiveeccentricity

Figure

20

Definition

of

eccentricity

b)

100%

overlap

joint

with

negative

eccentricity

X

joints

I nd Y

joints

/

/

N

and

K

joints

with

gap

N

andK

oints

with

overlap

I

/

oy[1%e2

-*

-

/

Figure 21

Joint

geometries

23

a

/

,es

—Y-------k-


33/100

8.

EXAMPLE

-

DESIGN

BRIEF AND APPROACH

8.1 Brief

The

client

requires

a

single storey, single bay

industrial

building

to be used as

a

ight

machine

shop.

It

is

to be

sited

on an

industrial

estate on

the

outskirts

of

Leicester.

Main dimensions

-

30

m

wide

x 48 m

long

x 6.7 m to

eaves

Cladding

-

Colour

coatedsteel

sheetsto

roof,

sides and

ends with

20%

natural

lighting provided

by

translucent

sheet

inserts.

Insulation

-

A

lining

system

to be

provided

to

wall

and roof

sheeting.

Access

-

A roller

shutter

door 4 m

X

4 m

is to be

provided

in both

gable

ends with

personnel

doors 1 m

x 2 m

adjacent

and

along

the

side

walls.

Note:

The

possibility

hat the roller shutter doors would

be

open

during

a

severe

storm was

discussed

with the

client.

The

final

decisionwas

that the

design

should

be based on the

assumption

that both

doors

would

be

closed

during

a

severe

storm.

Services

-

Allowance

was to be madeto

support

set-vices

from the

roof

structure. Mechanical

handling

was not

required.

General

-

It

was

agreed that:

(a)

The roof

pitch

wouldbe set at50

(b)

The roof to be of hotrolled hollow section

latticed

framework.

(c)

Hot rolled

I

sections

wouldbe usedfor the

columns.

The outlineof the

building

basedon the above brief is shown

in

Figure

22.

Selection

of

RHS

for

the

roof

structure

is

based

on its

enhanced efficiency

and

the

cost

effectiveness of

oints

which

will,

in

general,

be

quite

simple.

The

girders

willbe

shop

fabricated

in

two

halves,

approximately

15

m in

length

and

1.2

m

deep.

Hollow sections

can be used

in

simple,

semi-rigid

and

rigid design

and

can

adequately

carry

axial

(tensile

and

compressive)

loads, bending,

shear and

torsion.

8.2

Cladding

Since the decision

has

already

been

made

to use

colour coated

steel

sheets with

insulation

lining

and translucent

sheet inserts,

it

is only

necessary

to

settle

uponthe most

suitable

thickness

and

profile

of

sheet to be

adopted.

This neednotbe the same for both

roof

and

sides and

they

are therefore

considered

separately.

24


34/100


35/100

8.2.1 Roof

sheeting

The

span

of the roof

s 30

m

and with

a 5°

pitch

the

length

of

one

slope

is

marginally

over

15 m. Not all manufacturers

producesheeting

of

such

length

and

it

may

be

necessary

to

use,

say,

2/8 m sheets

lapped

at the centre. The

laps

should be bedded in sealant becauseof the

low rise.

A suitable

spacing

for the

purlins

will be 1.85

m,

whichon a

slope length

ofabout

15

m,

dividesthe rafter

of

he

roof

frame into

eightpanels.

A

typical sheetingsystem

would

be

the

"Warmclad

1000R",

with

lining,

manufactured

as

a

BSC Profile

(Reference

6),

this

is

suitable

for roofand walls.

8.2.2

Wall

sheeting

The

height

from

the floor

to the eaves

is

6

m

hence

sheeting

rails can be

spaced

at

1.5

m

c/c.

To

achieve

a

differentarchitectural effect

to

the

building

either

a

different sheet and/or an

alternativecolour couldbe

adopted.

Figure

23

shows

a cross

section

of the

building.

6.0

m

0.7m

,1

Figure

23

Cross section

26

Lined

roof

cladding

and translucent sheets Ridge

tie

Lined wall

cladding

'Weathering

curb

ground

evel

15.0 m

Half

span

design for structural steel work for frame industrial building

Documents