London 2012 Olympic Ceremonies - Video Screens

Proceedings of the International Association for

Shell and Spatial Structures (IASS) Symposium 2013 „BEYOND THE LIMITS OF MAN”

23-27 September, Wroclaw University of Technology, Poland

J.B. Obrębski and R. Tarczewski (eds.)

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London 2012 Olympic Ceremonies – Video Screens

Piers Shepperd1, Jeremy Lloyd

2, Rastislav Bartek

3, Stefano Cammelli

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1Technical Director, London 2012 Ceremonies, London, UK, [email protected] 2 Technical Design and Staging Manager, London 2012 Ceremonies, London, UK, [email protected]

3 Associate Structural Engineer, Buro Happold, London, UK, [email protected] 4 Head of Civil Structures, BMT Fluid Mechanics, Teddington, UK, [email protected]

Summary: This paper describes how four ultra-large LED video screens were suspended from the London 2012 Stadium roof in order to enhance the

spectator experience in the Stadium and for the TV audience at home during the London 2012 Olympic and Paralympic Ceremonies. Each video screen was 19.2m wide x 7.68m high and suspended 39m above the ground, which made for an excellent viewing experience for everybody in the

stadium.

Keywords: non-linear buckling analysis, retractable fabric membrane, wind tunnel testing, parametric modelling, temporary structures

1. INTRODUCTION

This paper describes the logistical and technical challenges behind just

one item of Olympic Ceremonies infrastructure - the four LED video

screens suspended from the Stadium lighting towers. It illustrates the

major technical and logistical challenges which had to be resolved by

the Ceremonies team, in order to provide these critical screens used to

deliver important video content for all four Ceremonies.

The screens were used to show films (e.g. the now famous James Bond

parody involving the Queen) which were an essential part of the show

and allowed spectators present at the stadium to see the videos broadcast

to the TV audience during the show [1].

The four LED screens represented a design challenge due to their weight

(10tonnes each) but also because of their large surface area exposed to

wind (19.2m x 7.68m). Supporting such large objects from the stadium

lighting towers was not anticipated in the original analysis and design of

the roof [2] and therefore the Ceremonies team faced a big challenge.

The creative team (led by Artistic Director Danny Boyle) wanted to

ensure the Stadium screens were the largest possible to enhance the

stadium spectator experience but keep within the engineering design

constraints.

These constraints included: (i) not exceeding the maximum load

allowance for the roof on both the local and global scale – the total

weight of all ceremonies objects supported from the stadium roof was

approximately 600tonnes, (ii) preventing significant reduction of pre-

stress in the cable system stabilizing the lighting towers and (iii) not

exceeding the global buckling capacity of the lighting tower legs.

Fig. 1. LED Video Screens in use during the London 2012 Olympic Opening Ceremony

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The “porous” Spider 30 LED screen product was used in an attempt to

reduce the wind loading. Following on the initial desk study based on

[3,4] the wind tunnel study was commissioned and performed in BMT

boundary layer wind tunnel in Teddington. Three different screen sizes

were tested to get a better understanding of the relationship between

wind pressures and screen surface area in parallel with different

configurations of blockage behind and below the screens. Blockage

scenarios included different states of deployment of a retractable fabric

membrane at the back of the screens and housings for projection and

follow spot equipment mounted above the roof edge.

In order to reduce weight, aluminium and high strength steel

components were used and the screen supporting frame was suspended

from the stadium lighting towers so that most of the primary elements

were in tension or combined tension and bending rather than

compression.

The traditional solution of solid panels mounted behind the screens for

improving contrast was replaced with light fabric membrane designed to

be retractable. This made the screen solid when deployed but porous

when retracted in case of wind speeds exceeding the agreed maximum

operational.

The final size of the screens was driven by the buckling capacity of the

stadium lighting tower legs and was determined based on complex,

detailed non-linear buckling analysis of the stadium roof model updated

with all Ceremonies loads including the loads acting on the screens.

Fig. 2. LED Video Screen – video testing during installation

2. OLYMPIC CEREMONIES OVERLAY

The 2012 London Olympic Stadium roof is a pre-stressed bicycle wheel type cable net clad with pre-stressed fabric membrane with additional

complexity of supporting 14 large lighting towers at the roof edge which

are stabilized by another pre-stressed bicycle wheel type cable system elevated 30m above the main roof [2]. Structurally it represents a

delicate balance between pre-stress forces and dead load.

During the original roof design, the Stadium roof design team consulted key people behind the technical aspects of previous Olympic

Ceremonies and therefore some Ceremonies roof overlay structures as

cable net supporting Ceremonies Scenery radial automation were

anticipated and allowed for in the design [2].

Other Ceremonies roof supported structures were not anticipated, they

represented significant additional load and extra pre-stress to the roof

cable net and therefore Ceremonies technical team joined up with the original roof designers to deliver them.

As shown on Fig. 3 the roof supported Ceremonies infrastructure

consisted of: primary and secondary radial automation cables, North-South high speed catenary automation, Audio and Lighting bicycle

wheel type cable net supporting Ceremonies Speaker clusters, 4 ultra

large LED video Screens suspended from lighting towers, roof platforms supporting Ceremonies technical equipment, equipment enclosures and

scenery props parked on the roof.

Fig.3 Ceremonies overlay/roof supported infrastructure (highlighted red)

This represented in total approximately 600 tonnes of additional weight

and as such significant and complex design load case for the Stadium roof structure. Apart from overall weight its distribution was also

challenging as big portion of it had to be located at the roof edge and all

scenery parts/objects of the load had dynamic components due to acceleration/deceleration and breaking/stopping of winches.

3. LED SCREENS DESIGN INTENT

3.1. Options and constraints

As mentioned earlier the driver for the large LED video screens from the

Creative Team side was to enable broadcasting of the video content essential to the show which would otherwise be only available to TV

audience also to spectators present at the Stadium. This was important

for life engagement with the spectators and therefore great atmosphere of the show.

Many options for the large LED video screens were considered with

screens located above or below the roof edge, with various sizes and various aspect ratios. Feasibility of these options was preliminarily

assessed from architectural/design point of view – the fit with the main

stage layout and storyboard, structural point of view, sight lines requirements, viewing angles and cost. Some options were also tested on

site using mock-ups.

3.2. Selected Option

Based on the design intent studies and preliminary assessment from the

technical design team it was concluded that:

i. The LED screens needed to be positioned approximately 4m

above the roof edge to maximize the number of spectators who

can see them. Also to allow smooth performers mobility along the roof edge gantry and to allow ceremonies equipment (lights,





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follow spots etc) to be positioned and operated in the zone

below the screen.

ii. The screens needed to be porous to maximize the surface area

and benefit from possible wind load reduction due to porosity.

iii. To achieve good video quality and contrast the porous screen needed to have black/dark background.

iv. In order to achieve both (ii) and (iii) objectives the dark

background needed to be designed as retractable. Maximum possible “operational” wind speed should be based on wind

tunnel results and detailed structural analysis.

v. The only locations where screens can be 3m above the roof tension ring and not in the way of radial scenery automation

cables Stadium lighting towers.

vi. The number of screens and the actual size would be determined based on structural analysis informed by wind tunnel testing

and the maximum possible size acceptable from the structural

point of view should be used – function of the additional weight and wind load imposed on the Stadium Lighting towers

3.3. LED screen product

Spider F-30 product [5] was chosen because it is modular and

lightweight and therefore offers flexibility in application, it uses high

quality SMD LED technology and XLed processors to create good video quality.

A single Spider F-30 tile is shown on Fig. 4. It is 960mm x 960mm

square tile with130mm depth, 50% transparency, 30mm pixel pitch, 120º h/v viewing angle and approximately 20kg/m2 weight.

Fig.4 Typical Spider 30 LED screen Tile

Comparatively low weight and 50% transparency were particularly

interesting from the structural point of view as the aim was to maximize

the screen area and minimize the weight. The actual porosity of the screen and its effect on reduction of wind loads was investigated in the

wind tunnel as described in more detail below.

The modularity of the product was advantageous for site installation and enabled tiles to be manually handled. Modularity also reduces risk of

technical failure as each tile is standalone unit and both electronics and individual LED strips can be easily replaced.

4. WIND LOADING

4.1. Desk Study

A desk study was initially conducted with the aim of quantifying the

peak wind loading acting on the video screens and assessing the level of

uncertainty and sensitivity to key design parameters / assumptions.

As part of this preliminary study, a detailed wind analysis was carried

out to determine the wind properties at the site. The wind analysis was

based on the widely accepted Deaves and Harris wind model of the atmospheric boundary layer, as defined in ESDU (Engineering Sciences

Data Unit) Item 01008, and has provided wind profiles describing the

variation of wind speed and turbulence intensity with height for a full range of wind directions. The wind analysis also took detailed account

of the variation of the up-wind terrain on each wind sector and initial

considerations on the possible speed-up factors over the roof of the stadium were also given.

An initial review of the recommendation given in the British Standard

suggested the use of an overall net pressure coefficient for solid signboards of 1.8, with an eccentricity of the force acting normal to the

screen of 0.25 its entire width. No indication of the variability of the net

pressure coefficient with aspect ratio of the signboard and no guidance on the potential alleviation of wind loading associated with porosity of

the screen were given within the code.

The examination of technical papers such as [3] and [4], as well as the review of the guidance given in the Australian/New Zealand Standard,

was able to provide the design team with insight knowledge on the drag

force coefficient for a range of rectangular signboards with varying aspect ratios and clearance ratios, for different levels of porosity and for

a range of angles of incidence. This review seemed to indicate that, for the desired size of the video screen the creative team had in mind, values

of overall net pressure coefficient in the region of 1.2 to 1.4 (depending

on the net effect of the porosity) were possible. Also, the eccentricity suggested in these references appeared to be lower than the one

specified in the British Standard.

Overall the above evidence showed that there was scope for a reduction in the wind loading initially assumed in design. The technical team felt

therefore that the best way to pursue and better quantify this reduction

was to perform a campaign of wind tunnel tests on a scaled model of the video screen.

4.2. Wind Tunnel testing

The main aim of the wind tunnel testing was to assess the mean force and torque coefficients of a number of different configurations of the

video screens. Specifically the design team was interested in:

- Quantifying the wind loading for different aspect ratios of the video screens (14, 21 and 28 LED modules in the horizontal dimension

and 8 along the vertical one);

- Quantifying the wind loading taking into account the presence of the structural system supporting the LED modules (namely the three

horizontal box trusses and the reinforced elements running

vertically) as well as ancillaries such as the ladders at the back of the screens;

- Quantifying the influence of blockage below the video screen;

- Quantifying the influence of the folded fabric in the center of the video screen (occupying ~10% of the total width of the screen

itself);

- Investigating and assessing any scale effect associated with gaps between the horizontal elements of each LED module;

The wind tunnel tests were conducted at BMT Fluid Mechanics, making

use of their large boundary layer wind tunnel facility. The test section of the wind tunnel is 4.8m wide, 2.4m high and 15m long with a 4.4m

diameter turntable and has an operating wind speed range of 0.2 - 45m/s.

The choice of the geometrical scale of the physical model (1:12) was dictated by the following considerations:

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- Desire to achieve a wind tunnel model large enough to allow a good

representation of the different details that are likely to affect the local and overall wind flows at full scale;

- Reduction of scale effects potentially associated with the flow going

through the gaps between the horizontal elements of each LED module;

- Minimization of the blockage ratio in the wind tunnel (kept to ~5%

in order to satisfy the experimental requirements of the Quality Assurance Manual of the Australasian Wind Engineering Society);

The wind tunnel model (see Fig. 1 and Fig. 2) was constructed

employing selective laser sintering (SLS), a manufacturing technology where a high power laser selectively fuses plastic powder by scanning

cross-sections generated from a 3-D digital CAD file, a process which is

able to achieve a high degree of accuracy (model scale tolerance of ±0.1mm). The wind tunnel model was mounted on a steel supporting

frame connected to a six components high-frequency force balance. In

addition, the lighting tower immediately behind the screen and the walkway situated beneath the screen were also modeled to account for

their influence on the flow around the porous screen.

The wind tunnel tests, conducted in smooth flow conditions, were run at a speed scale of 1:1, leaving only a factor of 12 between the model scale

and the full scale Reynolds number regime. For each tested

configuration, the measurements of mean force and torque coefficients were taken for a 180 degrees sector (between wind into the front to wind

into the back of the video screen) at 22.5 degrees increments. The

aerodynamic wind load coefficients were obtained by subtracting from the total loading acting on the video screens and the supporting frame

employed in the wind tunnel as a supporting device, the loading acting on the bare supporting frame only (i.e. with no video screens in place).

Considerations on the effect of turbulence / boundary layer flow on the

measured coefficients were given.

The measured mean force and torque coefficients (CFx normal to the

screen, CFy in the plane of the screen and CMz torque about the center-

line of the screen), including the effect of the folded fabric at the back of the screen, are presented in Table 1.

Table 1: Wind tunnel test results: mean force and torque coefficients

Component 28 Panels 21 Panels 14 Panels

CFx [-] 1.31 1.25 1.20

CFy [-] 0.31 0.31 0.30

CMz [-] 0.04 0.06 0.05

Although the measurements obtained appear to be relatively well

aligned with the ones suggested in the technical references considered during the early stage of the project, a direct comparison - for example -

against the results presented by Letchford is not straightforward; the

main features making this comparison difficult are:

- Difference in wind flow regime between the two experiments

(smooth flow conditions in the case study here presented vs.

turbulent boundary layer flow in the work conducted by Letchford);

- Difference in the porosity of the screens tested: uniform in the

experiments conducted by Letchford and - due to the presence of

trusses, vertical supporting elements, ladders and the folded fabric in the center of the video screen - highly non-uniform in the case

study here presented;

- Aerodynamic interference, in the case study here presented, with the

lighting tower immediately behind the screen;

It should be noted that, due to the complexity of the actual geometry of the video screens, conducting wind tunnel testing in atmospheric

boundary layer with a manageable mismatch of integral length scales

would have been practically impossible.

The peak wind loads were subsequently obtained making use of the

quasi-steady assumption, according to which the peak forces acting on the video screen can be predicted by using the measured mean force

coefficients with an appropriate duration of peak gust wind speed (in

this specific case in the range of ~3s to ~5s, depending on the aspect ratio / overall size of the screen itself).

Fig. 5. Wind tunnel model – overall view

Fig. 6. Wind tunnel model – close-up view

5. STRUCTURAL ANALYSIS

5.1. Design Methodology

The principle of the structural engineering task in this case was about

finding a solution with maximum surface area of the screen and minimal weight such that the screen frame itself satisfied all strength and

serviceability limits and more importantly it did not compromise

structural behaviour of the Stadium roof or Lighting towers and associated steelwork and cable connections.





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The structural design process was automated by modeling the screen

frame parametrically in Rhino/Grasshopper and linking the Grasshopper definition automatically with structural analysis software (Oasys GSA)

using custom Grasshopper components.

To inform this process with as precise loading information as possible 3 different screen sizes were tested in the wind tunnel – the 14x8 LED

tiles, 21x8 LED tiles and 28x8 LED tiles. The wind loading for

intermediate sizes was linearly interpolated.

With the height (vertical dimension) of the screen being the same the

only principal parameter which was variable was the screen width.

For each screen width which was considered the structural design and analysis of the screen frame was performed, followed by global buckling

analysis of the lighting towers and re-analysis of the stadium roof cable

net in order to check impact on the pre-stress in cables.

Based on results of this parametric study a 20x8 LED tiles screen was

selected as the largest acceptable from the structural point of view.

Also a result of this study a slightly different solution for the screen frame supporting the 20x8 tiles case was derived as the one used in the

wind tunnel testing. More efficient, easier to fabricate and install on site,

lighter as the middle support truss/member was eliminated and representing less blockage for the air flow through the screen.

The bigger screens represented a problem from structural point of view

– especially impact on stability of lighting towers – but also from site installation point of view as the extra weight lead to requirement for

bigger size chain hoists which were not available within the short

procurement program. The weight of the wider screens was driven by cantilever portions of the screen frame projecting outside from the

inclined lighting tower legs.

The individual steps of the analysis process and the key results are

described in more detail below.

5.2. Structural frame forming the LED video screen

The design of the structural frame forming the LED screens was driven

by three main factors: (i) minimization of self-weight, (ii) simple fabrication and (iii) installation out of small parts which can ideally be

manually handled.

The key factor for minimization of frame weight was the retractable fabric drape as the frame design wind load was reduced to wind load

acting on porous LED panel.

Additionally the decision was made to design the screens as suspended (top hung) rather than bottom supported as that allowed the support

beams (highlighted blue on Fig. 7) to be in combined tension and

bending and comparatively slender as if in compression their size would be governed by buckling.

Fig. 7 shows the geometry model of the screen frame. The screen is

suspended on 4No. 12mm diameter Pfeifer galvanized spiral strand cables (red) – in reality almost invisible. The top edge of the screen

frame is formed by square box section (green) which resists wind load

from top half of the screen and also supports screen hanger beams holding the LED tiles.

The lateral support of the bottom edge is provided by the walkway truss

(purple). In order to minimize the truss weight the diagonal members were omitted and 4mm aluminium decking was designed to replace

them. The front boom of the walkway truss is laterally restrained by

screen hanger beams and the back boom by another 4 No.hanger cables and a handrail/edge protection truss (highlighted orange).

Fig.7 Geometry model of the 20x8 LED tiles screen support frame

(fabric drape, LED tiles, mechanical parts are not shown for clarity)

All structural elements (apart from the main beam supporting the top

edge) were designed either out of alluminium or high strength steel

(originally the whole frame was designed out of alluminium but due to very short fabrication program and associated availability constraints

some elements were replaced with high strength steel elements of similar weight).

5.3. Connection of the LED screen to lighting towers

In order to reduce the installation time and to minimize impact on

Lighting towers the connections of LED screens to existing structure

were designed such that only 4 bolts per screen had to be site drilled and the rest of connections were designed either in compression bearing or

as clamps.

The top hanger beams (magenta colour on Fig. 7) are fully supporting the screen weight and were designed to accommodate simultaneously

the temporary hanger cables for lifting beam and permanent hanger

cables which replaced the temporary ones at the end of installation. The top hanger beams are projecting from the front face of the lighting

towers to allow straight vertical lift of the screens during installation and

to allow minimum distance between the back face of the screen frame and lighting tower legs required for the maintenance (in case some work

on LED electronics or mechanical components of the retractable screen

drape would be required).

The two lower connections of the screen frame to lighting tower legs

(light blue on Fig. 7, red on Fig.8) are providing lateral support for the

screen frame but also are connecting the two tubes forming lighting tower leg together in order to engage both in resisting the lateral wind

load acting on the screens. Connection to front tube only would lead to

buckling failure of the lighting towers.

The two lower connections are designed as clamps around the lighting

tower leg tubes to simplify the installation and dismantling.

All connections to lighting towers were designed to accommodate adjustments to allow screen frame to be installed level and symmetric to

ensure that both legs of the lighting tower are equally loaded.

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Fig.8 Lower connection of the screen frame to lighting tower leg

5.4. Effects on the existing roof and lighting towers

The 20x8 LED tiles video screen assembly weighs approximately

10tonnes which is equal to 30% of lighting tower weight. The screen is

suspended from first access platform (cross members connecting lighting towers legs) and therefore lighting tower legs are subject to

significant additional load.

The total wind load acting on the lighting tower with the screen is 1.8 times higher than total wind load acting on lighting tower without screen

which is very significant increase and is adding significant additional

bending moments into the lighting tower legs and increases lateral displacements.

This meant that even more detailed global buckling/stability analysis

had to be undertaken than the one during the original design of the lighting towers. Additionally the roof cable net had to be completely re-

analysed and re-checked with all Ceremonies overlay loads including

LED video screen dead loads. Roof deflections (and ponding) had to be checked, forces in main radial suspension cables and verification had to

be done that the lower radial cables and lighting tower backstay cables

do not lose pre-stress due to the significant additional weight of screens and significant additional lateral wind load.

5.4.1. Global Stability/Buckling of Lighting Towers

In order to estimate the critical buckling load of Stadium Lighting

Towers supporting LED Video Screens and associated additional loads the following analyses were performed:

- Linear (eigenvalue) buckling analysis

- Geometrically non-linear buckling analysis with/without initial

imperfection

- Geometrically non-linear elastic-plastic buckling analysis

with/without initial imperfection

Linear (eigenvalue) buckling analysis

The linear buckling analysis has many limitations and is potentially un-conservative but is relatively easy to perform and therefore is a good

starting point. The 2012 Olympic Stadium roof is a pre-stressed

mechanism with large displacements and non-linear behavior. Therefore it cannot be analysed using linear structural analysis. This was reflected

in the main roof analysis model (used for the design of the stadium roof)

where the cable net was modeled as a mechanism, containing elements such as cables and fabric membrane (1D and 2D tension only elements)

which can only be solved using incremental non-linear analysis. For the

purpose of performing linear eigenvalue buckling analysis the model had to be “linearised” - lighting towers were isolated together with

backstay and circumferential cables from the primary cable net roof

model. The stiffness of the primary cable net supporting lighting towers was represented by elastic spring supports. The lighting tower bases

were modeled as “pinned” with elastic translational springs in the X, Y

and Z directions. The elastic spring stiffness at every major tension ring

node location (lighting tower supports) was derived from the main analysis model of the roof. The stiffness at each location is a function of

geometry of the major radial cables, their pre-stresses and local stiffness

of the compression truss.

Non-linear buckling analysis

The linear buckling analysis, using the geometry and loading of the

initial configuration of the structure, gave only an indication of the

buckling load within the elastic limits of the material. It assumed linear

correspondence between the applied load and the resulting

displacements of the structure. It did not consider imperfections and did

not provide any information on post-buckling behavior.

Therefore non-linear analysis had to be performed. Two types of non-

linearity were considered in the global buckling analysis of lighting

towers – the geometric non-linearity and material non-linearity. The

geometric non-linearity considered non-linear relationship between the

applied load and resulting deformation. The material non-linearity

considered plasticity of structural steel.

Firstly just the geometric non-linear analysis was performed to

investigate elastic instability. This was followed by analysis considering both geometric and material non-linearity (elastic/plastic analysis) to

investigate whether there is plastic behavior before buckling instability

occurs. Definition of the non-linear material model used in analysis (bi-linear stress/strain curve and tri-linear moment rotation diagram) can be

found on Fig. 9 and Fig.10. The worst case of plastic collapse was

assumed considering hardening parameter equal to zero.

Fig.9. Bi-linear stress/strain curve – elastic/perfectly plastic material

Fig.10. Tri-linear moment vs. rotation curve – initial/partial/full yielding





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The tri-linear moment/rotation relationship of members purely in

bending was assumed in the elastic/plastic non-linear analysis of lighting

towers. Section behaves elastically until limiting elastic moment

capacity is reached (yield stress in extreme fiber). This state is

automatically reported by the analysis software. Then rotational stiffness

is reduced and section is partially plastic until plastic moment capacity

of the section is reached. When section reaches the full plasticity the

rotational stiffness is zero. Locations of plastic hinges are also

automatically reported by the analysis software. The effect of plasticity

on the interaction between axial force and biaxial bending is considered

automatically. The net bending moment is limited to the plastic capacity

of the section for the current axial force in the member.

In order to check influence of the fabrication and erection tolerances on

the global behavior and stability of lighting towers all checks were

repeated using the model with the initial geometry updated with the

initial geometrical imperfection.

Two types of initial geometrical imperfections were used – in order to

cover imperfection in-plane and out-of-plane of the lighting tower (refer

to Figure below):

- Type 1 – 1st buckling shape (eigenvector), normalised and scaled in

accordance with BS EN 1993-1-1:2005, 5.3. The amplitude of

110mm which is L/250 was used.

- Type 2 - Deflected shape (due to wind acting on the tower and the

LED screen), normalised and scaled in accordance with BS EN

1993-1-1:2005, 5.3. The amplitude of this initial imperfection

represents approximately 30% of total deflection due to SLS load combination which is significant initial eccentricity from perfect

geometry.

Fig.11. Initial geometrical imperfection – Type 1 (top) and Type 2

(bottom)

Effect of connection stiffness on the global stability

Stadium lighting tower frame contains many welded CHS connections

between tubes with different diameters. Some stiffened with gusset

plates (where required from local capacity point of view) and others

without any stiffening. Although there was no doubt about local capacity

of connections and their components their stiffness represented

uncertainty in the global stability analysis assumptions.

In order to verify sensitivity of global stability of lighting towers with

LED Screens to the rotational stiffness of the connections all above

mentioned analyses were repeated with different assumptions about

connections.

Selected results of the global buckling analyses (displacement vs. load

factor diagrams) are summarised in the graphs on Fig.12 and 13.

From the graphs on Fig. 12 it is clear that the rotational stiffness of

connections has significant influence on the global stability of the

Stadium lighting towers. Two scenarios are presented – fully moment

resistant joints and semi rigid joints. The reality is between the two but

most likely close to the “semi rigid” scenario.

The global buckling factor was established between 1.5 -1.8xULS load

[1.2DL+1.4WL] on the tower with the screen – depending on the

connection stiffness (most likely close to 1.5). The buckling factor

derived from linear buckling analysis was 5.2 which shows that the

linear buckling analysis is not adequate for this type of structures.

Fig.12. Results of non-linear buckling analyses – displacement vs. load

factor diagrams – load factor as a multiple of [1.2DL + Wind]

If the global buckling factor was represented as multiple of ULS load

where 1.2xDLremains constant (the dead load of Lighting towers and

LED screens is known with high certainty and was verified on site) and

just wind loading was incrementally increased (more realistic scenario)

then the global factor is higher – around 2.3-2.4 (refer to Fig. 12 –

graphs highlighted black).

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Fig.13. Influence of initial geometric imperfection

The Fig. 13 shows the influence of initial geometric imperfections on the

global stability analysis results. The influence is clearly visible but is not

very significant.

5.4.2. Effects on the Stadium cable net roof and the Lighting

towers stabilising cables

Fig.4 shows comparison of calculated vertical deflections at the roof

edge due to Ceremonies loads in comparison with theoretical design

snow loads. As can be seen the two loads (which were assumed as not coexistent) are causing comparable roof deflections and in locations

where lighting towers are supporting the screens the theoretical roof

edge deflection due to Ceremonies loads is even higher (highlighted red on Fig. 4).

Fig.14 Roof edge vertical deflection due to Ceremonies load and

theoretical design snow load – for comparison.

This was obviously cause for concern as snow load in original design of

the Stadium roof (in combination with dead load) was the critical case for ponding (maintaining positive roof slope/drainage away from the

FOP). Also it was the critical/limiting case for maintain the Lighting

tower stabilizing cables (backstays) stressed at all times (obviously in factored conditions).

However the critical areas for above mentioned issues in the original

roof design were at the East and West side of the roof and as can be seen from Fig. 4 in these areas the deflections due to Ceremonies Loads are

lower than the deflections due to snow load which means the original

design was not compromised. This shows that controlling the distribution of the Ceremonies roof load was as important as controlling

of its magnitude.

In order to make sure that the total weight would not get any higher it was agreed that the weight of all individual Ceremonies objects and

equipment was verified and recorded prior to installation on site.

5.4.3. Operational wind speed

The maximum operational wind speed for the fabric masking drape was

derived such that the total wind load acting on the screen with fully deployed masking drape (function of the operational wind speed) was

made equal to the total design wind load acting on the porous screen

with retracted fabric drape (based on the design wind speed).

The fabric masking drape and associated mechanical components were

then designed for the maximum operational wind load.The actual

operational wind speed was set slightly lower at 15m/s (3s gust).

Approximately one year before the beginning of the London 2012

Olympic and Paralympic Games, five wireless weather stations were installed within the Olympic Stadium with the purpose of continuously

monitor ambient conditions; wind speed (with up to a 3-s gust

resolution) and wind direction were among the quantities measured, processed and reported on a monthly basis;

Two of these stations (see Fig. 15) were installed on Lighting tower legs

at height relatively close to the height of the geometric center of the video screens and therefore able to provide good indication of the local

mean wind speed under the aerodynamic influence of the roof of the

stadium itself (the so-called ‘speed-up’ factor).

For full-scale mean wind speeds locally approaching ~8-10m/s the local

wind speed-up factors (in the range of 1.05 - 1.10) measured at the site

were found to be in relatively good agreement with what observed in a previous campaign of (smaller scale) atmospheric boundary layer wind

tunnel testing conducted on the stadium itself.

These stations were monitored all the time during the Olympic Ceremonies and Olympic Games and when the measured wind speed

reached specified operational wind speed the masking drape behind the

LED video screens was retracted.

Fig. 15. Anemometer mounted at the lighting tower access platform

(LED Screens level)





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6. RETRACTABLE MASKING DRAPE

As mentioned earlier in order to achieve high video quality during the day time (early evening hours) there is need of having dark background

behind the Spider F-30 LED product as otherwise the 50% transparency

allows light to come from behind and reduces the contrast.

For wind load related reasons (explained earlier) it was decided that the

dark background was designed as retractable. In order to make it as

lightweight as possible decision was made to design it as fabric membrane – hence the name retractable masking drape.

Fig.16.Testing of the deployment of the fabric drape

The retractable mechanism is shown on Fig. 17. The mechanism driven by double drum winch (green) linked to two lines – the deploying line

highlighted in red and retracting line highlighted blue.

Both lines are connected to the drape top edge aluminium lifting beam. Each line starts as single line and is then split into 4 parallel lines in

order to pull the lifting beam up/down uniformly.

The actual fabric drape is made of single layer PU coated Polyester base cloth spanning horizontally approx.1m between vertical sliding tracks

mounted at the back face of screen hanger beams.

Ronstan 19mm sliding tracks were used together with Ronstan sliding cars attached to fabric membrane.

Fabric membrane was reinforced with 30mm x 8mmGRP battens

positioned parallel with top edge lifting beam in fabric pockets at approx. 1m centres “linking” the Ronstan sliding cars also attached to

fabric in these locations.

The battens were most important during retracting when fabric membrane was not stressed and battens were supporting it laterally

making it heavier and stiffer to make sure that it slide down along the

rails. Deploying phase is much easier as fabric is being stressed and the uniformity along the screen width is achieved by stiffness of the

aluminium lifting beam.

Retractable mechanism was tested during the installation. It was used few times during the Olympic Games as it was specified that it must be

retracted in case of measured wind speed being higher than specified

operational wind speed and during the night when Stadium was not

used.

Fig.17. Retractable mechanism

7. FABRICATION

The program for the procurement and fabrication of screen frames and

retractable drape was very tight. Selected contractors and Ceremonies

Technical Team were facing a challenging 5 weeks between May and June 2012.

To achieve this on such tight schedule the fabrication/procurement

contract was split into three main packages:

- The supply of 640No. Spider F-30 LED tiles by Mediatec

- Fabrication and site assembly of screen frames by Wi Creations

- Fabrication and site installation of retractable drape by Banks Sails

In order to ensure that everything fits and goes smoothly on site a 3D

model was created by Wi Creations in Autodesk Inventor integrating all packages and containing all available data from fabrication model of As

Built Lighting towers, screen frame, all mechanical components

associated with retractable fabric drape to fabrication model for lifting beam and temporary hangers.

8. SITE INSTALLATION

The Ceremonies LED screens are big and heavy objects with big surface

area exposed to wind and therefore it was critical that the installation

method was considered in detail during the design stage and that the

design of the LED screens support frame considered the site installation

requirements/constraints.

Main issues influencing the choice of installation method were the

following:

- Time/Access. The installation of LED screens was planned for end

of June and beginning of July 2012 (4 weeks before the Opening

Ceremony) at which point heavy crane access to the field of play

(FOP) was very limited.

- Audio and Lighting cable net (pre-stressed bicycle wheel cable net

suspended from the roof tension ring supporting 28No. 2tonne

speaker clusters required for Ceremonies) was already in place which represented the following two complications

o A&L tension ring is 25m offset from the existing lighting

towers on plan towards the center of FOP and therefore the very

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large crane would have been required to allow lift of the screen

over it.

o There is always one radial A&L cable connected to the existing

roof tension ring in between the lighting tower legs which

prevents vertical lift of the video screen as a single unit. Because of this radial cable the screen had to be lifted in two

halves and had to be designed with splices at mid span allowing

this to happen.

- Only 2T chain hoists were available and therefore each half of the

screens have to be lifted with 3 chain hoists.

All the above mentioned constraints were considered. Detailed structural

analysis of the installation sequence was performed and a temporary

lifting beam was designed and fabricated which was initially suspended

from the LED screens support beams which allowed connection of 6No

2T chain hoists required for vertical lift of the screen in two halves.

The screen frame was designed out of small elements which could be

manually handled which made the assembly in the stadium seating area

easier. Also this allowed adding the weight of the LED screen on

lighting tower incrementally – row by row which was good from the

structural point of view and allowed monitoring of the tower and the

roof during this process for any unexpected/unusual signs of distortion.

The LED screens were initially fully assembled at the ground level and

the retractable drape was added/installed and tested (easy access).

Fig. 18. Screen assembled row by row suspended on 6 chain hoists. Video testing while easily accessible at upper tier.

The actual screen support frame had splices in all elements allowing the

screen to be split (following the video electronics and drape mechanism

testing) and lifted in 2 halves. The two halves were lifted one by one as

vertical position of the screen halves does not affect uniformity of the

load applied onto the lighting tower.

Once the screen was lifted above the Ceremonies Audio and Lighting

cable net the two halves were connected. The access to the splice joints

was provided from crane basket of small crane which could still access

the Stadium FOP at that stage. Once the halves were connected the

screen was lifted into final level. At that stage the permanent hangers

were installed, screen was leveled and connected to lateral supports and

finally disconnected from chain hoists. The screen installation was

seamlessly executed by Unusual Rigging.

Fig. 19. Lifting of half of the screen using 3No 2tonne chain hoists.

Fig. 20. Connecting of the two halves once both lifted above the Audio

and Lighting cable net.

9. REFERENCES

[1] Moles S, Isles of Wonder, Lighting and Sound, Issue 313, 09/2012

[2] Crockford I. et al., The London 2012 Olympic Stadium: Part 3, The

Detailed Design and Construction of the Roof, IABSE/ IASS

Conference “Taller. Lighter. Longer”, 09/2011, London, UK

[3] Warnitchai et al.,Wind Tunnel Model Tests of Large Billboards,

Advances in Structural Engineering, Vol. 12, No. 1 (2009)

[4] Letchford., Wind loads on rectangular signboards and hoardings, J. of Wind Eng. and Industrial Aerodynamics, Vol. 89 (2001)

[5] BS 6399-2: 1997 Loading for buildings - Part 2: Code of practice

for wind loads

[6] AS/NZS 1170.2:2002 Structural design actions - Part 2: Wind

actions

[7] Australasian Wind Engineering Society, AWES-QAM-1-2011, Quality Assurance Manual - Wind Engineering Studies of

Buildings

[8] ESDU (Engineering Science Data Unit) Item 01008, Computer program for wind speeds and turbulence properties: flat or hilly

sites in terrain with roughness

London 2012 Olympic Ceremonies - Video Screens

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