07 - notional loading

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It is dened as m =

wherem is the number o columns in a row

that are connected to the bracing system

being considered. These columns must also

be supporting at least 50% o the average

vertical load o those columns in the row being

considered (Figure 1):

Clause 5.3.(4)B in Eurocode 3-1 states that

where the overall applied lateral load is more

than 15% o the vertical load in a member then

the notional horizontal load can be ignored. This

is expressed as H

�d ≥ 0.15 V�d in the above

reerenced clause.

Designprinciples

A notional load is based on a proportion o

the vertical load the structure is supporting.

Typically they are applied in conjunction with

other loads during analysis.

Generic Notional Horizontal Load(F hn)Eurocode 1-1-6 concerns loading during the

construction o structures. Within Annex

A, Clause A1.3 o Eurocode 1-1-6 there is a

generic denition o a notional horizontal load

(F hn) that can be applied to all structures.

The magnitude o this orce is 3% o the

vertical loads rom the worst case load

combination or a given structure. This can be

adopted or all structures, regardless o the

material they have been constructed rom.

Material Sensitivity to Notional LoadNotional loads represent orces that come

about due to imperections in the structure.

Some materials are more sensitive to this

phenomena than others and it is or this

reason that notional loads are linked directly to

the material a structure is constructed rom.

The Eurocodes or steel and concrete

structures have sections within them that

are dedicated to deriving notional horizontal

loads within structures. The ollowing sections

explain how each material addresses notionalloading.

Introduction

This Technical Guidance Note concerns the concept o notional loading,

which the Eurocodes classifes as Equivalent Horizontal Forces. These are

loads that exist due to inaccuracies and imperections introduced into the

structure during its construction. The ollowing text explains how notional

lateral loads are incorporated into the design process.

All o the guides in this series have an icon based navigation system, designed

to aid the reader.

Notional

loading

• Design principles

• Applied practice

• Worked example

• Further reading

• Web resources

Notional Loads in Steel FramesSteel rames are very sensitive to notional

loads. This is because imperections within

the abricated elements and their connections

are inevitable as they are impactul. It is or

this reason that any design o a steel rame

structure must take them into account.

Eurocode 3-1-1, Clause 5.3.(3) covers this

by creating coefcient (ф ), which the vertical

load o a structure is multiplied by. This

replaces (F hn) notional load rom Eurocode

1-1-6 described previously.

Coefcient (ф ) is determined thus:

ф = ф 0 h m (Equation 5.5, Eurocode 3-1)

Where:

ф 0 is the sway angle at which the structure

rotates due to notional loads and has a base

value o 1/00

h is the actor that is related to the height o

vertical elements within the structure.

This is dened as h = ,

where ‘h’ is the height o the structure.

This actor can only be within the range

o 0.66 < h <1.0. I the calculated value

lies outside o this bracket, then the closer

extreme is taken.

m is the actor that takes into account thenumber o vertical elements in a row.

ICONLEGEND

March 2012

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March 2012

Notional Loads in Concrete FramesThe Eurocode’s approach to imperections o

elements within concrete structures is very

similar to the one adopted or steel ramed

structures. The only key dierence is that

there is some provision or horizontally alignedelements as well as those that are vertical.

Eurocode -1-1 Clause 5.(5) denes the

notional load coefcient (Ѳ)in a similar way

to Eurocode 3-1-1 in that it is a unction o the

vertical load the structure is supporting. There

is however a slight dierence to the derivation

o (Ѳ) to take into account the length o

elements as well as their height:

Ѳ = Ѳ0 h m (Equation 5.1, Eurocode -1)

Where:

Ѳ0

is dened as the sway angle o the

structure, in a similar ashion to Eurocode

3-1-1, described above.

h is the actor that is related to the height or

length o vertical elements within the structure.

This is dened as h =

where ‘l ’ is the length or height o members.

This actor can only be within the range o

0.66 < h <1.0. I the calculated value lies

outside o this range, then the closer extreme

is taken.

m =

as per steelwork structures - with the value

om varying in accordance with the extent

to which the structure is being analysed. For

isolated members, the value o m is 1, while

or braced rames m is the number o vertical

elements contributing to a braced rame. I a

oor slab is being assessed, the value o m is

the number o vertical members contributing

to the horizontal orce that is exerted onto the

oor slab.

Notional Loads in Isolated Elements within Concrete and Steel FramesIt is possible to assess the impact that

notional loading has on isolated elements

within structures. Clause 5.(7) in Eurocode

-1-1 describes two dierent methods o

assessing the impact o imperections on

individual elements within a structure. Either

can be employed, but once a method has

been selected, it should be used exclusively

throughout the project.

Method (a) considers the eccentricity o

elements as they are constructed. This isdened as: ei = ф l /2 or steel rames or

ei = Ѳl /2 in the case o concrete rames.

Where:

ei is the eccentricity

ф /Ѳ is the angle o rotation due to theapplication o the notional load as per steel

and concrete ramed structures

l 0 is the length o the element

When considering a wall or an isolated column

within a braced structure, ei can be estimated

to be l 0 /400.

Once the value o ei is determined, it is

multiplied by the maximum axial load o the

member being considered.

Method (b) imposes a lateral orce,H i

onto

the element at a point along the element that

generates the maximum bending moment

rom this load. Typically this is at the mid-

span position.

For members that are not within a braced

rame, the orce H i is dened as ф N or ѲN ,

whereN is the total axial orce and ф /Ѳ

is dened above. For elements within the

braced rame the value o H i is 2ф N or 2ѲN ,

depending on the structure’s material.

Partial Factors for Notional LoadsNotional loads are considered in combination

with applied lateral loads, such as wind. The

partial actors applied to them reect that

they exist within the structure prior to any

load being applied to it. They are treated in the

same way as a wind load and are classied as

a variable static action within the Eurocodes.

When notional loads are combined with only

the dead and imposed loads, they typically

adopt the ψ combination actor, which is

0.5. When used in combination with the wind

load, the combination actor ψ is applied

(typically 0.).

Here is an example o how notional

horizontal loads would be combined into

a single load case when the imposed load

(Qk,) is the leading variable action or a

commercial ofce building:

1.35Gk + 1.5Qk, + (0.7ψ)1.5Qk, +(0.2ψ)1.5Qk,3Where Gk is the dead load, Qk, is the imposed

load, Qk, is the wind load, and Qk,3 is the

notional load. The combination actor or the

wind load is ψ and the combination actor or

the notional load is

ψ. Combination actors

can vary depending upon the type o use othe building when the wind load is taken as

the primary action. See the UK National

Annex to Eurocode 0, Table NA.A1.1 or the

applicable actors.

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Appliedpractice

The applicable codes o practice or the

derivation o notional loads are as ollows:

BS En 1991-1-6 Erocode 1: Actions on

structures — Part 1-6: General actions —

Actions during execution

BS En 1991-1-6 uK ntio aex to

Erocode 1: Actions on structures — Part

1-6: General actions — Actions during

execution

BS En 1992-1-1 Erocode 2: Design o

reinorced concrete structures —Part 1-1:

General rules and rules or buildings


Erocode 2: Design o reinorced concrete

structures —Part 1-1: General rules and rules

or buildings

BS En 1993-1-1 Erocode 3: Design o steel

structures —Part 1-1: General rules and rules

or buildings


Erocode 3: Design o steel structures

—Part 1-1: General rules and rules or

buildings

Glossary andfurther reading

actio – An applied load, both due to a

direct application or as a consequence o an

indirect eect such as thermal expansion o

the structure.

accidet actio – A loading condition

that is unlikely to occur. As such partial

actors are not applied to it during ULS

analysis.

Chrcteristic od – A base load that

has not had any partial actors applied to it.

ntio aex – A part o the Eurocode

that has been written specically or a

particular region.

notio od – A load that exists within

the structure due to imperections that

cause a lack-o-t.



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Prti fctor – A actor that is applied

to characteristic loads when carrying out

design o structures and the elements they

are constructed rom.

vribe sttic ctio – A load that is

static, yet variable. Notional loads are typical

o this type o action.

Further ReadingManual or the design o steelwork building

structures to Eurocode 3 – Institution o

Structural Engineers – October 010

Web resources

For more inormation on this subject, please

visit: www.istrcte.org/resorces-cetre/

ibrry

igur 2 Isomtric viw o proposd commrcial

building

With the value o ( ф ) calculated, the magnitude o the notional horizontal load

can be calculated:

Worked exampleInitially the need to include notional loading within the analysis o the

structure is checked. This is done by comparing the applied wind load on a

vertical element against 15% o the axial load, thus:

Now that the need or the inclusion o notional loading has been proven to be

positive, actor ( ф ) needs to be calculated.

A 5 storey commercial property is to beconstructed rom a steel rame structure.

It has a 10m by 8m grid layout and the wind

load upon it is 1 kN/m2. The internal columns

have an axial load o .5 MN and all edge

columns have 1.5MN. Corner columns

have an axial load o 0.75MN. The structure

is braced via a pair o concrete lit shat

and stair cores. Figure shows the overall

dimensions o the structure.

Determine whether or not notional loads

should be applied to this structure and i

so, what their magnitude is. This should be

carried out or all orthogonal directions in

accordance with good practice.

07 - notional loading

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