j.p. jaspart, j.f. demonceau and l. comeliau - …...robustness of structures – codes and...

1|ROBUSTNESS OF STEEL STRUCTURES

European Erasmus Mundus Master Course Sustainable Constructions under Natural Hazards and Catastrophic Events

Robustness of structures – Codes and standards J.-F. Demonceau & J.-P. Jaspart, ULg

Standards and design guides

J.P. JASPART, J.F. DEMONCEAU and L. COMELIAU

Timisoara, April 10, 2014




Introduction

�Since the progressive collapse of the Ronan Point tower

in 1968, some codes and standards have included some

recommendations to limit the risk of occurrence of

progressive collapse of buildings, for instance:

• The British Standards

• Canadian codes

• The Unified Facilities Criteria of the US Department

of Defence

• The Eurocodes...




Introduction

� The first code which introduced recommendations to

ensure the structural integrity of buildings was the BS

(BS5950-1)

�The other codes are mainly inspired from this code

(i.e. the same design philosophies are met)

�Within this presentation, two codes will be addressed:

• The British Standards (BS5950-1)

• The Eurocodes (EC1 Part 1-7)




Introduction



(BS5950-1)



�Within this presentation, two codes will be addressed:

• The British Standards (BS5950-1)

• The Eurocodes (EC1 Part 1-7)




British Standards BS5950-1Part 1: Code of practice for design – Rolled and

welded sections




Introduction �BS5950-1 dedicated to the structural use of

steelwork in building (Part 1: Code of practice

for design – Rolled and welded sections)

�§ 2.4.5 of this code, entitled “Structural

integrity”, gives recommendations to reduce

the risk of localized damage spreading




Introduction

�For this purpose, two main methods are

recommended:

• Tying of buildings (� indirect method)

• Key elements (� direct method)

�Using these methods, it may be assumed that

substantial permanent deformation of members and

their connections is acceptable

� SLS has not to be considered at this stage




Tying of buildings

Each column should be effectively held in position by

means of horizontal ties in two directions (approximately

at right angles) at each principal floor level supported by

that column

�Allow ensuring a continuity between horizontal

elements

�Allow a redistribution of loads within floor levels in

case of a column loss




Tying of buildings




Tying of buildings

The horizontal ties may be:

• Steel members, including those also used for other

purposes

• Steel bar reinforcement (anchored to the steel frame and

embedded in concrete)

• Steel mesh reinforcement in a composite slab with steel

sheeting designed to act compositely with steel beams

(steel sheet directly connected to the beam by shear

connectors)




Tying of buildings

�All horizontal ties, and all horizontal members, should

be capable of resisting a factored tensile load (not

additive to other loads) of not less than 75kN

� No information on the origin of this value !!!




Avoidance of disproportion. collapse �Where regulations stipulate that certain buildings

should be specially designed to avoid

disproportionate collapse, steel framed respecting

the design recommendation previously mentioned

may be assumed to meet this requirement provided

that five conditions are met




Avoidance of disproportion. collapse �Condition 1: general tying

• Horizontal ties should be distributed throughout each floor

as previously described

• Steel member acting as horizontal ties (and their end

connections) should resist to:

0,5.(1,4.gk + 1,6.qk).st.L > 75kN for internal ties

0,25.(1,4.gk + 1,6.qk).st.L > 75kN for edge ties

• This can be assumed to be satisfied if the member is capable

of resisting a tensile force equal to its end reaction under

factor loads (> 75kN)




Avoidance of disproportion. collapse




Avoidance of disproportion. collapse �Condition 2: tying of edge columns

• Horizontal ties anchoring the columns nearest to the

edges of a floor should be capable of resisting a

factored tensile load, acting perpendicular to the

edge, equal to the greater of:

o the load previously defined in “Condition 1”

o 1% of the maximum factored vertical dead and

imposed load in the column adjacent to that level




Avoidance of disproportion. collapse �Condition 3: continuity of columns

• All column should be carried through

at each beam-to-column joints

• All column splices should be capable

resisting a tensile load equal to the

largest factored vertical dead and

imposed load reaction applied to the

column at a single floor level located

between the column splice and the

next column splice down

Column splice




Avoidance of disproportion. collapse �Condition 4: resistance to horizontal forces

• Braced bays or other systems for resisting horizontal forces

should be distributed throughout the building

� Efficient bracing system

�Condition 5: heavy floor units

• Where precast concrete or other heavy floor units are used,

they should be effectively anchored in the direction of their

span

� To avoid a slipping out of these elements from their

supports




Avoidance of disproportion. collapse

�If any of the first three conditions (i.e. general tying,

tying of edge elements and continuity of columns) are

not met

�the building should be checked in each storey in turn

to ensure that disproportionate collapse would not be

precipitated by the notional removal, one at a time, of

each column




Avoidance of disproportion. collapse �If condition 4 (i.e. Resistance to horizontal forces) is

not met

� the building should be checked in each storey in

turn to ensure that disproportionate collapse would

not be precipitated by the notional removal, one at a

time, of each element of the systems providing

resistance to horizontal forces




Avoidance of disproportion. collapse �The portion of the building at risk of collapse should

not exceed 15% of the floor area or 70m² (whichever is

less) at the relevant level and at one immediately

adjoining floor.

�If it is not the case, the column or element supposed to

be lost should be designed as a key element!




Avoidance of disproportion. collapse �Loads to be considered for the checks for notional

removal of members:

• 1/3 of the ordinary wind load (W)

• 1/3 of the ordinary imposed load (Q)

• Dead load (G)

� 0,33.W + 0,33.Q + G

(except for storage or where the imposed load is of a

permanent nature)




Key elements�A member that is recommended to be designed as a

key element according to the previously presented

rules should be designed for the accidental loading

specified in BS6399-1

�BS6399-1: When an accidental load is required for a

key or protected element approach to design, that load

shall be taken as 34kN/m²

�Other structural elements providing lateral restraint

vital for the stability of a key element should also be

designed as a key element




Key elements�Loads to be considered for the key element design �

the same than the ones previously mentioned, i.e.:

• 1/3 of the ordinary wind load (W)

• 1/3 of the ordinary imposed load (Q)

• Dead load (G)

� 0,33.W + 0,33.Q + G

(except for storage or where the imposed load is of a

permanent nature)




Eurocode 1 Part 1-7Actions on Structures - General Actions - Accidental

actions




Introduction �EN 1991-1-7 provides strategies and rules for

safeguarding buildings and other civil engineering

works against identifiable and unidentifiable

accidental actions

�The recommended strategies range from measures to

prevent or reduce the accidental action to the design

of the structure to sustain the action




Introduction

�Principle 3.3.(1)P in EN1991 Part 1-7: Accidental

actions:

“In the design, the potential damage to the

structure arising from an unspecified cause shall

be minimised, taking into account its use and

exposure”




Introduction �Strategies to be considered:




Limiting the extend of localised failure �The potential damage to the structure arising from an

unspecified cause shall be minimised through one or

more of the following strategies:

• Designing the structure with enhanced redundancy

• Designing key elements

• Applying prescriptive design/detailing rules




�The strategies for

accidental design

situations may be based

on the Consequence

Classes:

• CC1 – Low consequences of

failure

• CC2 – Medium

consequences of failure

• CC3 – High consequences of

failure

Limiting the extend of localised failure





31





32





33





34




�For CC1

�No specific consideration for accidental actions

�For CC2

�Simplified analysis by static equivalent action models or;

�use of prescriptive design/detailing rules

�For CC3

�An examination of the specific case should be carried out to

determine the level of reliability required





�For CC1


�For CC2



�For CC3







Consequences Class 2�Lower Group

• Effective horizontal ties or effective anchorage of suspended

floors to wall should be provided

�Upper Group

• Effective horizontal ties together with effective vertical ties

should be provided or;

• To check the building to ensure that upon the notional

removal of each supporting column and each beam

supporting a column the building remains stable and that

any local damage does not exceed a certain limit




Consequences Class 2�Upper Group

• Limit of admissible local damage: 15% of the floor or 200 m²,

whichever is greater (?), in each two adjacent storeys

• For the elements that the removal induces a non-admissible

local damage � to be designed as a key element




Effective horizontal ties – Framed structures

�To be provided:

• Around the perimeter of each floor and roof level

and;

• Internally in two right angle directions to tie the

column and wall elements

�The ties should be continuous





�Effective horizontal ties may comprise:

• Rolled steel sections

• Steel bar reinforcement in concrete slabs

• Steel mesh reinforcement and profile steel sheeting in

composite steel/concrete floors (if directly connected to the

steel beams with shear connectors)





41





�Effective horizontal ties may comprise:

• Rolled steel sections

• Steel bar reinforcement in concrete slabs

• Steel mesh reinforcement and profile steel sheeting in

composite steel/concrete floors (if directly connected to the

steel beams with shear connectors)

�The ties may consist of a combination of the above

types

(Identical to what was recommended in the BS)





�Each continuous tie, including its end connections,

should be capable of sustaining a design tensile load:

- s is the spacing of ties

- L is the span of the considered tie

- Ψ is the factor according the

accidental load combination

i k k

p k k

For internal ties: T 0, 8.(g .q ).s.L or 75 kN

For perimeter ties: T 0, 4.(g .q ).s.L or 75 kN

= + ψ= + ψ




Effective vertical ties – Framed structures

�In framed buildings, the columns and

walls should be capable of resisting an

accidental design tensile force equal

to the largest design vertical

permanent and variable load reaction

applied to the column from any one

storey (not applied with normal

loading).

Column splice

R1

R2




Effective horizontal ties – Wall structures

�Lower group:

• To adopt a cellular form of construction

• Appropriate means of anchoring the floor to the walls

�Upper group:

• Continuous effective horizontal ties should be provided in

the floors

45





�Loads to be supported by the ties:+ ψ

=

=

t k k

i t

p t

F (g .q ) zFor internal ties: T max[F ; . ] kN/m

7,5 5For edge ties: T F kN/m

46





�Loads to be supported by the ties:

- Ft is the minimum between 60kN/m and 20 + 4ns with ns

the number of storeys

+ ψ=

=

k k

i

tp

t

t

(g .q ) zFor internal ties: T max[ ; . ] kN/m

7,5 5For edge ties: T

FF

F kN/m

47





�Loads to be supported by the ties :

- Ft is the minimum between 60kN/m and 20 + 4ns with ns

the number of storeys

- z is the minimum between the greatest distance of the

supporting elements and 5 times the clear storey height H

+ ψ=

=

t k k

i t

p t

F (g .q )For internal ties: T max[F ; . ] kN/m

7,5 5For edge ties: T F

z

kN/m

48





49




Effective vertical ties – Wall structures

�For masonry walls: a minimum thickness of 150 mm

and a minimum resistance to compression of 5N/mm²

is required

�H < 20t with t the thickness of the wall

�The vertical tensile load to be supported is:

- A is the cross-section of the wall in mm²

(excluding the non-load bearing leaf of a cavity wall)

= 234A HT max[ ( ) N ; 100 kN/m]

8000 t

50




Key elements

�A key element should be capable of sustaining an

accidental load Ad applied in horizontal and vertical

directions (in one direction at a time)

�Recommended value for Ad: 34 kN/m²





�For CC1


�For CC2



�For CC3






Consequences Class 3�A systematic risk assessment of the building should be

undertaken taking into account all the normal hazards

that may reasonably be foreseen, together with any

abnormal hazards




Consequences Class 3




Introduction �Strategies to be considered:




Specific action - Impact

�Conservatively, it may be assumed that only the

impacting object absorbs all energy

�For structural design purposes, the impact may be

represented by equivalent static forces (to check the

static equilibrium, the strength and the deformations)




Specific action - Impact

�Substructures for which the energy is mainly

dissipated by the impacting object:

Recommended minimum

equivalent static design

forces

Fd,x and Fd,y need not be

considered simultaneously

h, location of the impact

force Fd ranges from 0,5 m

(cars) to 1,5 m (lorries)




Specific action - Impact �For superstructures for which the energy is mainly

dissipated by the impacting object:

Recommended minimum

equivalent static design

forces

In specific cases, a Fd,y force

may be considered




Summary




Summary



(BS5950-1)



� Clearly demonstrated within this presentation for the

Eurocodes




Summary� In the Eurocodes, two main strategies are addressed:

• Strategies based on identified accidental actions

• Strategies based on limiting the extend of localised damage

�Within the first part of the exercise, different methods

recommended in the Eurocodes will be applied to a

steel building:

• The Tying method

• The Key element method:

oConsidering the impact of a vehicle

oConsidering the loss of a column

j.p. jaspart, j.f. demonceau and l. comeliau - …...robustness of structures – codes and...

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