Facade Design
Facade Design
Facade Design
BASIS OF DESIGN
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Table of Contents
Section 1 - INTRODUCTION ........................................................................................................................... 6
Section 1 - INTRODUCTION
1.1 Object of the Report
The purpose of this basis of design for facade structural engineering of the project LUSAIL TOWERS is to present:
o Load combinations
According project specifications [2], the codes listed here below will be used for the design of facades.
[6] BS 6399-1: Loading for buildings — Part 1: Code of practice for dead and imposed loads
[8] BS EN 1991-1-1: Actions on structures - Part 1-1: General actions - Densities, self-weight, imposed loads
for buildings
[10] ASCE 7-16: Minimum Design Loads for Buildings and Other Structures (2005)
[11] ANSI/ AISC 303-10– Code of Standard Practice for Steel Buildings and Bridges
[13] BS EN 1993-1-1: Design of steel structures - Part 1-1: General rules and rules for buildings
[15] AA – The Aluminum Design Manual, 2010 Edition, The Aluminium Association
[16] BS EN 1999-1-1: Design of aluminium structures - Part 1-1: General structural rules
[17] Center for Windows and Cladding Technology (CWCT) - Standard for systemized building envelopes
[18] ASTM E 1300-12a: Standard Practice for Determining Load Resistance of Glass in Buildings (2012)
[20] ETAG 002 - Guideline for European Technical Approval for Structural sealant
[21] BS EN 1808 - Safety requirements on suspended access equipment – Design calculations, Stability
criteria, Construction - Tests
All Eurocodes (EN), if any, listed here above are used with the British national annex (BS/NA).
MS Excel
Mathcad 15
S 275 (fu = 430 N/mm², fy = 275 N/mm², E=210 000 N/mm², =0.3)
S 355 (fu = 510 N/mm², fy = 355 N/mm², E=210 000 N/mm², =0.3)
1.4404 (AISI 316L) (fu = 530 N/mm², fy = 220 N/mm², E=200 000 N/mm², =0.3)
4.2 Aluminium
For bracketry:
Alloy EN AW 6082 T6 (E=70 000 N/mm², =0.3)
For cladding:
Alloy EN AW 5083 H111 (E=70 000 N/mm², =0.3)
4.3 Glass
Any equivalent materials according American standards could also be selected. In any case, the design will be
done according codes and standards related to the material chosen.
The permanent loads take into account self-weight of structures and finishing loads.
The following horizontal barrier loads are computed according the most stringent of the following codes:
Therefore, the façade elements shall be designed to sustain safely each of the following static loads acting
separately.
Horizontal uniformly distributed line load of 0.74 kN/m applied at 1.2m above FFL
Surface load of 1.0 kN/m² below the level +1.1 m above FFL
Horizontal uniformly distributed line load of 1.50 kN/m applied at 1.2m above FFL
Surface load of 1.50 kN/m² below the level +1.1 m above FFL
Following the “Center for Windows and Cladding Technology (CWCT) – Standard for building envelopes” Part 2
§ 2.3.3. Cleaning from ladders or cradles: Horizontal load of 0.50 kN acting on a square surface of 100 mm side.
Curtain Wall framing and anchorages are designed to withstand a BMU load of 2.0 kN acting in any directions,
as per specifications [2].
This load has to be coordinated/confirmed with façade access engineer at a later stage.
BMU loads will be cumulated with reduced wind loads (basic wind speed of 20 m/s, 3sec-gust) as per BS EN 1808
[21] (see load combinations in chapter 0). The suspended platform cannot be used during extreme wind.
A sand load of 0.6 kN/m² is considered on horizontal surfaces (such as the roofs) to take into account the
accumulation of sand on those surfaces.
The façade elements have to accommodate the movement due to thermal actions as per specifications:
T0 30 Reference temperature
For the cladding wind load assessment, a design wind speed of 38 m/s, 3 second gust at 10 meters high in open
terrain was used, as specified in the QCS 2014 (return period of 50 years).
The wind pressures given in the following are LRFD wind pressure (as per ASCE 7-16) with a return period of
1700 years. For an ASD design (or a design according BS Eurocodes), the wind pressures need to be multiplied
by 0.66 to be in accordance with a return period of 100 years.
The wind pressures mentioned in the following and in the table here above are applicable for tributary area of
1 m². A reduction factor can be applied on those wind pressures for higher tributary areas:
The maximum peak net negative and positive pressures (suction) on the façade walls and podium roofs and
soffits are given in the following figures.
The lad combinations here below are derived from ASCE 7-16 (in case of design according AISC or AA).
6.1.1 ASD
ASD/Serviceability combinations
Maintenance
Dead loads Live loads Wind loads (*) Sand loads (**)
Loads combinations loads (BMU,…)
(DL) (LL) (WL) (SL)
(ML)
ASD 1 1 1 - - -
ASD 2 1 - 0.66 - -
ASD 3 1 - - 1 -
ASD 4 1 0.75 0.495 0.75 -
ASD 5 1 - 0.16 (***) - 1
(*) Wind loads given in the cladding wind report have to be multiplied by 0.66.
(**) Sand loads are considered as the same than live loads. The sand loads are applied only on horizontal
surfaces.
(***) Factor 1.0 x 0.16 = 0.16 is applied on wind loads (wind speed of 20 m/s)
6.1.2 LRFD
LRFD combinations
Maintenance
Dead loads Live loads Wind loads (*) Sand loads (**)
Loads combinations loads (BMU,…)
(DL) (LL) (WL) (SL)
(ML)
LRFD 1 1.4 - - - -
LRFD 2 1.2 1.6 0.5 1.0 -
LRFD 3 1.2 1.0 1.0 1.0 -
LRFD 4 1.2 1.0 0.5 1.6 -
LRFD 5 1.2 1.0 0.26 (***) 1.0 1.6
(*) Wind loads given in the cladding wind report are LRFD wind loads (return period of 1700 years).
(**) Sand loads are considered as the same than live loads. The sand loads are applied only on horizontal
surfaces.
(***) Factor 1.6 x 0.16 = 0.26 is applied on wind loads (wind speed of 20 m/s)
BS EN 1990 and its national annex are applied for defining the load combinations at Ultimate Limit States and
Serviceability Limit States (in case of material selected according EN).
The table here below gives the ψ factors according BS EN 1990/ NA to be applied for accompanying actions:
As mentioned previously, Maintenance loads (BMU, suspended platform) will be combined with a reduced wind.
According EN 1808, this reduced wind is based on a basic wind speed of 20 m/s, 3 sec-gust. Therefore, a
reduction factor will be applied on the wind pressures when BMU loads are the leading components in the load
combinations below.
(*) Wind loads given in the cladding wind report have to be multiplied by 0.66.
(**) For sand loads, ψ factors are considered as the same as for wind loads. The sand loads are applied only on
horizontal surfaces.
(***) Factor 1.0 x 0.16 = 0.16 is applied on wind loads (wind speed of 20 m/s)
(*) Wind loads given in the cladding wind report are already ULS wind loads.
(**) For sand loads, ψ factors are considered as the same as for wind loads. The sand loads are applied only on
horizontal surfaces.
(***) Factor 1.5 x 0.16 = 0.24 is applied on wind loads (wind speed of 20 m/s)
The factored efforts are from the load combinations defined in the previous chapter (according BS EN if selected
structural materials are according BS EN or according American Standards – ASCE if selected structural materials
are according AISC or AA).
The design strength capacity is determined according partial strength method for BS EN:
𝑅
𝑅 =
𝛾
Where 𝑅 is the characteristic resistance and 𝛾 are material partial safety factors.
The design strength capacity is determined with a reduction factor for American standards:
𝑅 = 𝑅 .𝜑
Where 𝑅 is the characteristic resistance and 𝜑 are strength reduction factors
7.2 Serviceability
The serviceability criterion are defined according basis of design [1]. In case of secondary steel or aluminium
structure, the design is according the corresponding standards as mentioned in chapter 7.1 about strength
design.
7.2.1 Claddings
The deflection of the infill panels will be limited so as not to introduce a permanent deformation with the design
pressure loads. The limit of the elastic stress is yield strength under characteristic design pressure loads.
Following the basis of design [1] and project specifications [2], the horizontal/vertical deflection of the transoms,
mullions and all facade elements in the plane of glazing will be limited to the following deflections.
The horizontal deflection parallel to the glazing plane will be limited to span/360 or 3.2 mm.
The vertical deflection of any main horizontal framing member due to imposed load will be limited to
span/360 or 3.2 mm.
Following the basis of design [1] and project specifications [2], the deflection of the framing structure (curtain
walls and claddings) will be limited to respect the following deflection :
Δ ≤ H/175 if H ≤ 4.11m
Δ ≤ H/240 + 6.35mm if H > 4.11m
The design of glazing is done according ASTM E1300-12a [18]. The chapter 24 of IBC 2009 [19] gives
recommendations for design of glass infill panels resisting to wind loads which are in line with design according
ASTM E1300:
As per IBC’s recommendations, the Allowable Strength Design (ASD) is used for glass design, the load
combinations of IBC will be therefore used for glazing design. The wind pressures provided by RWDI are
computed with a basic wind speed with a return period of 1700 years and shall be therefore multiplied by 0.66.
The climatic loads (internal pressures of IGU) are considered as a live load.
The load duration of each load combination are based on the load duration of the main load case acting in the
load combinations.
The load resistance is computed with a probability of breakage (Pb) of 8 lites or plies of 1000.
The allowable stresses are presented here below depending on the load duration. Those allowable stresses are
in line with the appendix X.6 of ASTM E1300:
The allowable deflection at centre of glass is limited to L/75 or 25 mm whichever is lesser, for a panel with a
shorter dimension smaller than 1.5 meters and L/120 + 7.6 mm for any other cases. It has to be noted that the
deflection is computed with a reduced wind loads (return period of 10 years – reduction factor of 0.34).
The section 6.10 of ASTM gives that procedure for a Double Glazed Insulating Glass (IG) with Monolithic Glass
Lites of Equal (Symmetric) or Different (Asymmetric) Glass Type and Thickness Simply Supported Continuously
Along Four Sides:
Where LR1 is the load resistance of lite 1 and LR2 is the load resistance for the lite 2.
NFL is the non-factored load and is given in the appendices of ASTM E1300. Here below is given as an
example the NFL for a glass of 8 mm thick.
GTF is the glass type factor depending if the glass is annealed, heat strengthened or fully tempered (see
table 2).
LS is the load share factor which is a multiplying factor derived from the load sharing between the
double glazing, of equal or different thicknesses and types in a sealed unit (see table 5).
The load resistance LR computed according ASTM E1300 can therefore be compared to the wind pressures give
in RWDI report.
For specific or non-current glazing (cases not supported by ASTM E1300), the design will be done by finite
element approach and following the recommendations of ASTM E1300 by limiting the deflection and the
stresses as per chapters 8.2 & 8.3 .
The calculation of structural sealant is done according ETAG-002 and supplier’s datasheets.
The external cladding shall be capable of accommodating the following movements without any reduction in
the specified performance:
The following main structure movements have an impact on façade design and has to be provided/ discussed
with the Structural Engineer:
The limits of displacements and movements of the primary structure are defined here below.
For the cladding design, the vertical deflection of slab edge has to be decomposed between vertical movement
before cladding installation (slab deflection during construction) and after cladding installation (slab deflection
under permanent loads at long term and live loads).
The vertical deflection of a slab/beam can be subdivided in different components as per the sketch here below:
Where:
w1 is the pre camber of the slab or the beams if any (this is not relevant for this project)
wb is the long term deflection under permanent loads (after cladding installation)
This is considered as the maximum deflection which can occur during the construction of the building. This
limitation is taken into account for the bracket design (see §9.3 ).
This is considered as the maximum deflection which can occur during the design life of the building.
The incremental deflection is the deflection that occurs after façade panels installation.
In curtain wall, both interlocking transom and mullion joints are required to accommodate the service
movements of the frame and maintain weather tightness. Where occupied floors are next to unoccupied floors
(or stiffer floors next to current floors), both opening and closing movements will occur.
The differential vertical deflection of the slab edges/ beams (between adjacent floors) has to be limited to
The inter storey drift (horizontal displacement at each floor) has to be limited. The total horizontal displacement
u is not relevant for façade design.
As per project basis of design [1], the allowable inter story drift under wind (return period of 25 years) is:
ℎ
𝑢≤
500
𝑙𝑖𝑚𝑖𝑡𝑒𝑑 𝑡𝑜 𝟏𝟐 𝒎𝒎 𝑜𝑛 𝑡ℎ𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑓𝑎𝑐𝑎𝑑𝑒 𝑝𝑎𝑛𝑒𝑙𝑠
The drift is tested according AAMA 501.4, with a design displacement as per above. After this test, the
performance in terms of air and watertightness has to be maintained. In the following, a test is performed
again with an horizontal displacement of 1.5 x design displacement in order to verify the non-disengagement
or detachment of the façade elements though weather performance may be compromised.
The elastic drift will be tested according AAMA 501.4. After this test, the performance in terms of air and
watertightness has to be maintained. In the following, a test is performed again with an horizontal
displacement of 1.5 x elastic displacement (as recommended in AAMA 501.4) in order to verify the non-
disengagement or detachment of the façade elements though weather performance may be compromised.
Note: Inter-storey drift testing will be done according to the most onerous criteria defined under wind or
seismic drift value.
The façade shall accommodate the axial shortening under dead loads at long term and under live loads. Those
movements has to be transmitted by the Structural Engineer to the Façade Engineer at a later stage.
In a first estimation, the remaining axial shortening after façade installation is considered as 1 mm per meter
high.
The movements which affect the curtain walls can be divided in two categories:
Movement which occurs during the service life (after cladding installation)
Those allowable/maximum deflection are combined as per the load combinations defined at 0. A ψ factor of 0.6
is applied on the thermal movement to consider this is an accompanying action.
For a floor height of 4.25 meters, this corresponds to an allowable movement in the horizontal stack joint of:
Closure movement: + 20 mm
A tolerance on the position (X,Y,Z) of the primary concrete structure of ± 25 mm is considered. This will be
considered in the design of curtain wall support brackets (see §9.3 ).
The tolerance in the bracket design have to take into account the tolerances of construction defined in §9.2 and
the vertical deflection occurring before installation of cladding (see §9.1.1 and §9.1.3):
X Y Vertical
Tolerances in mm ± 25 ± 25 +25/-30
A vertical tolerance of +25/-30 mm is sufficient to accommodate the slab edge deflection during construction
and before cladding installation.
1.8 W/m²K for vision part of the curtain wall (including frames)
The thermal design will be performed with BISCO software developed by PHYSIBEL.