Verification of Insulating Glass Units in modern Curtain Wall Facades

Florian Döbbel, Sika Services AG

Michael Elstner, AGC Interpane

Avoiding Surprises …

▪ Review and comments on details

▪ Evaluation of glass stress and probability of breakage

▪ Deflection and thermal stress resistance of the specified glass configurations

▪ Indication of guidelines and recommendations for proper handling and installation as well as the protection

of glass after installation

▪ Revision of external shading, internal shading devices and certain glazing systems that may cause

excessive thermal loading on the glass resulting in potential glass breakage

▪ Analysis of the shop and / or architectural drawings to determine if glass requires heat-treatment.

Design Review, et. al.

Reference: GANA Glazing Guideline 50th Anniversary

Factors influencing glass type and thickness selection

acc. ASTM E 1300 -16

Reference: Wörner/ Schneider/ Fink – „Glasbau“, Springer Verlag

Glass Strength = Resistance to break

▪ Degree of damage and size of the surface

▪ Speed and duration of the impact

▪ Time span between surface damage and impact

▪ Environmental conditions

▪ Stress distribution

▪ Glass Composition

▪ Residual stress of glass

Influences on Resistance

.LR q ;LSF GTFNFLLR iiiii

The basic procedure in ASTM E1300

▪ … uniform lateral load of short or long duration …

▪ … rectangular shape with continuous lateral support …

Scope of ASTM E 1300

Shapes and Loads …

and new construction types

Line Loads

Concentrated

Loads

What counts, if a standard solution isn’t existing?

Specifications

▪ Wind loads, both positive (inward) and negative (outward)

▪ Dead loads

▪ Thermal loads

▪ Snow/Ice loads

▪ Impact loads

▪ Seismic loads

▪ Interior pressure from HVAC equipment

▪ Interior pressure due to building stack effect (these tend to be inward at the base of a building and outward near

the top of the building)

▪ Live loads (generally glass is not designed for people to walk on it; however occasionally design professionals

specify a live load to account for a distribute load that may be applied for maintenance)

▪ Typically there is an additional loads if insulating glass units are used. ASTM E 1300 is using a load share

factor between the lites of an IGU, but does not address the internal “climatic” loads.

Design Loads

Reference: GANA Glazing Guideline 50th Anniversary

…and we have a „special system“ Insulating Glass Units – IGU´s

const.T

p

IGU effect

Load Sharing

0

20

40

60

80

100

2000x3000 1000x2000 1000x1000 700x1000 500x500

Pro

port

iona

l loa

d ou

ter

lite

[%]

Dimension glass unit axb [mm]

standard load sharing [1][4]

ratio of glass thickness (3)

Feldmeier (5)

Outer lite: 10mmCavity: 12mmInner lite: 8mm

0

20

40

60

80

100

2000x3000 1000x2000 1000x1000 700x1000 500x500

Pro

port

iona

l loa

d ou

ter

lite

[%]

Dimension glass unit axb [mm]

standard load sharing [1][4]

ratio of glass thickness (3)

Feldmeier (5)

Outer lite: 8mmCavity: 12mmInner lite: 10mm

Calculating the characteristic edge length

*aa1

14

Btt

ttt

p

Ea* 4

v

3

2

3

1

cavity

3

2

3

1

B

Load Sharing due to the IGU Effect

Calculating the IGU factor

Calculating the stiffness of the outer (t1) and inner lite (t2)

tt

t3

1

3

1

3

11

- 1

tt

t13

2

3

2

3

22

Load Sharing

Loading Load direction Load component taken by the outer lite Load component taken by the inner lite

Negative wind pressure Pressure onto the inner lite

Positive wind pressure Pressure onto the outer lite

Both panes isochric pressure Both panes

totaln,11, w 1w n totaln,212, w w n

totalp,211, w w p totalp,22, w 1w p

op - op

Why should we consider „climatic“ effects?

▪ Multiple external reflections caused by concave and convex deformation of the individual lites

▪ Small and narrow and special shaped glass types lead to a different stress levels of the glass which can be

underestimated compared to wind loads

▪ Curved glass units – Increased climatic loads due to the stiffness the internal loads

▪ Stress in the secondary sealing bite is also influenced from the internal loads of an IGU

Climatic Effect - Influencing Factors

Principle

▪ Internal load effect

▪ Expansion or contraction of the gas volume enclosed into the hermetically sealed cavity

External Variations

• ΔT - Temperature difference between manufacture and use

• Δpmet - Difference between barometric pressure at site of manufacture and site of installation

• ΔH - Difference of altitude between site of manufacture and site of installation

Variations due the glass make up

▪ Thickness of outer and inner lite

▪ Dimension of the glass unit

▪ Dimension of the cavity

Reference: JRC-Report „Guidance for European Structural Design of Glass Components”

Changes in Temperature

3 K 1 kPa

Changes in Barometric Pressure

10 mbar = 1kPa

83 m 1 kPa

Unfavourable Superposition

Isochoric Pressure p0

.kPa/m012.0H pkPa/K34.0Tp atm0

1. 1.4D

2. 1.2D + 1.6L + 0.5(Lr or S or R)

3. 1.2D + 1.6 (Lr or S or R) + (L or 0.5W)

4. 1.2D + 1.0W + L + 0.5(Lr or S or R)

5. 1.2D + 1.0E + L + 0.2S

6. 0.9D + 1.0W

7. 0.9D + 1.0E

Combining Factored Loads

Using Strength Design

1. D

2. D + L

3. D + (Lr or S or R)

4. D + 0.75L + 0.75(Lr or S or R)

5. D + (0.6W or 0.7E)

6a. D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)

6b. D + 0.75L + 0.75(0.7E) + 0.75S

7. 0.6D + 0.6W

8. 0.6D + 0.7E

Combining Nominal Loads Using

Allowable Stress Design

Reference: ASCE/SEI 7-10: Minimum Design Loads for Buildings and other Structures,

American Society of Civil Engineers, Structural Engineering Institute, 2010.”

Load Case 1 to 4 are very often decissive for large sizes

(edge length > 500 mm / 700 mm; aspect ration > 1:3)

Load Case 8 to 12 are very often decissive for large sizes

(edge length > 500 mm / 700 mm; aspect ration > 1:3)

Load Combinations

.

n

2.6X],2[,llowablea

allowable

3/d]n,d[

Calculating the „allowable“ stress

Basic design stress for annealed glass, 3s load duration is 23.3 Mpa

Basic design stress for heat strengthened glass, 3s load duration is 46.6 Mpa

Basic design stress for fully toughened glass, 3s load duration is 93.1 Mpa

LOAD DURATION 3 S 60 S 600 S

(10 MIN)43 200 S

(12 HOURS)473 040 000 S

(>> 1 YEAR)

[MPA] [PSI] [MPA] [PSI] [MPA] [PSI] [MPA] [PSI] [MPA] [PSI]

AN 23.30 3 380 19.32 2 803 16.73 2 427 12.81 1 858 7.16 1 039

HS 46.60 6 750 42.44 6 147 39.49 5 720 34.55 5 004 25.84 3 742

HS, FRITTED 28.00 4 050 25.46 3 688 23.69 3 432 20.73 3 003 15.50 2 245

FT 93.20 13 500 87.54 12 683 83.46 12 089 76.35 11 059 62.90 9 111

FT, FRITTED 55.90 8 100 52.52 7 610 50.08 7 254 45.81 6 635 37.74 5 466

Load duration and “allowable” stresses

Case study: 33 Tehama, San Francisco, CA, USA

© Hines Constructions & Invesco

Case study: 33 Tehama, San Francisco, CA, USA

Boundary conditions taken into account for glass verification of the spandrel units GD-6:

• Production site (Lauenfoerde, Germany): 97 m [318 ft] a.s.l.

• Installation site (San Francisco, CA, US): 5 m [16 ft] a.s.l.

• Installation height: 123 m [420 ft]

• GD-6: Spandrel unit, insulating glass make-up o Outer lite: 6 mm [1/4 in.] Planibel Clearlite ipasol Neutral 48/27 # 2, fully tempered & heat

soaked o Space: 20 mm [3/4 in.] air, aluminum spacer, black o Inner lite: 6 mm [1/4 in.] Planibel Clearlite, fritted, RAL 7035 # 4, fully tempered & heat soaked

• Dimensions (width x height) o Smaller span: 997 mm x 391 mm [39.3 in. x 15.4 in.] o Maximum span: 1505 mm x 3452 mm [59.3 in. x 135.9 in.]

• Wind load (3 sec): -2.155 kPa / +1.915 kPa [-45 psf / +40 psf]

• Continuously simply supported

• Internal shadow box make up o Internal backup: 63 mm [2 ½ in.] distance, air, not ventilated o Insulation: 50 mm [2 in.], R = 1,43 m

2K/W

o Steel back pan: 1 mm [1/32 in.] o 200 mm [7 7/8 in.] air, not ventilated, R = 0.18 m

2K/W acc. to ISO 6946

o 20 mm [3/4 in.] sub ceiling or floor, R = 0.1 m2K/W

• Isochoric pressure

o Climatic effects, summer (Tcavity ≤ 80K; patm ≥ -2kPa; Haltitude ≤ 30m): +29.6 kPa [+618 psf]

o Climatic effects, winter (Tcavity ≥ -24K; patm ≤ 4kPa; Haltitude ≥ 15m): -12.0 kPa [-251 psf]

Results

principle

stress

allowable

stress

utilization deflection allowable

deflection

limitation ratio principle

stress

allowable

stress

utilization deflection allowable

deflection

limitation ratio

[MPa] [MPa] [%] [mm] [mm] [%] [MPa] [MPa] [%] [mm] [mm] [%]

LC1* 24.8 93.20 27 -24.8 25.4 98 25.2 93.20 27 -25.1 25.4 99

LC2* 24.7 93.20 27 -24.7 25.4 97 25.2 93.20 27 -25.2 25.4 99

LC3* 23.5 93.20 25 22.7 25.4 89 23.5 55.90 42 22.7 25.4 89

LC4* 23.5 93.20 25 22.8 25.4 90 23.5 55.90 42 22.6 25.4 89

LC5* 24.4 93.20 26 -24.8 25.4 98 17.8 93.20 19 -16.1 25.4 63

LC6* 19.8 93.20 21 -18.8 25.4 74 22.9 93.20 25 -22.7 25.4 89

LC7* 16.0 93.20 17 13.7 25.4 54 23.3 55.90 42 22.8 25.4 90

LC8* 21.4 93.20 23 20.5 25.4 81 18.6 55.90 33 16.9 25.4 67

LC9 6.3 76.35 8 -6.6 25.4 26 6.3 45.81 14 6.6 25.4 26

LC10 2.6 76.35 3 2.7 25.4 11 2.6 76.35 3 -2.7 25.4 11

LC11 0.1 62.90 0 -0.1 25.4 0 0.1 37.74 0 0.1 25.4 0

LC12 0.0 62.90 0 0.0 25.4 0 0.0 37.74 0 0.0 25.4 0

* non-linear FE Analysis

Load

case

Outer glass Inner glass

GD-06: Spandrel, Maximum Span

Glass dimension: 1505mm x 3452mm; Glass make-up: 6mm FT & HST / 20mm Air / 6mm FT & HST, fritted on #4

Wind load (ULS, 3sec): -2.155 / +1.915 kPa Climatic effects, summer: 29.6 kPa

Climatic effects, winter: -12.0 kPa

principle

stress

allowable

stress

utilization deflection allowable

deflection

limitation ratio principle

stress

allowable

stress

utilization deflection allowable

deflection

limitation ratio

[MPa] [MPa] [%] [mm] [mm] [%] [MPa] [MPa] [%] [mm] [mm] [%]

LC1 2.2 93.20 2 -0.2 7.8 2 4.4 93.20 5 -0.3 7.8 4

LC2 1.6 93.20 2 -0.1 7.8 2 5.0 93.20 5 -0.4 7.8 5

LC3 3.9 93.20 4 0.3 7.8 4 2.0 55.90 4 0.1 7.8 2

LC4 4.4 93.20 5 0.3 7.8 4 1.5 55.90 3 0.1 7.8 1

LC5 35.7 93.20 38 -2.5 7.8 33 30.7 55.90 55 2.2 7.8 28

LC6 12.6 93.20 14 0.9 7.8 12 17.6 93.20 19 -1.3 7.8 16

LC7 31.1 93.20 33 -2.2 7.8 28 35.6 55.90 64 2.5 7.8 32

LC8 17.2 93.20 18 1.2 7.8 16 12.7 93.20 14 -0.9 7.8 12

LC9 45.8 76.35 60 -3.3 7.8 42 45.8 45.81 100 3.3 7.8 42

LC10 18.5 76.35 24 1.3 7.8 17 18.5 76.35 24 -1.3 7.8 17

LC11 0.6 62.90 1 0.0 7.8 1 0.6 37.74 1 0.0 7.8 1

LC12 0.0 62.90 0 0.0 7.8 0 0.0 37.74 0 0.0 7.8 0

Load

case

Outer glass Inner glass

GD-06: Spandrel, Smaller Span

Glass dimension: 997mm x 391mm; Glass make-up: 6mm FT & HST / 20mm Air / 6mm FT & HST, fritted on #4

Wind load (ULS, 3sec): -2.155 / +1.915 kPa Climatic effects, summer: 29.6 kPa

Climatic effects, winter: -12.0 kPa

GLASS TYPE STEP OF COMPARISON FAILURE PREDICTION MODEL FPM ALLOWABLE STRESS DESIGN ASD

SMALLER SPAN997 MM X 391 MM [39.3 IN. X 15.4 IN.]

6 MM [1/4 IN.] FT & HST20 MM [3/4 IN.] AIR

6 MM [1/4 IN.] FT & HST

STEP 1LOAD SHARING ACC. TO [2]

NEGATIVE WIND LOAD: -2.155 KPA [-45 PSF]

LSF1 = LSF2 = 2.00NFL = 11.1 KPA

GTF1 = GTF2 = 3.6LR = 11.1 KPA X 3.6 X 2.00

LR = 79.92 KPA

LSF1 = LSF2 = 2.00SMAX = 3.62 MPA

SALLOW,1 = SALLOW,2 = 93.2 MPA *SALLOW,A,1 = SALLOW,A,2 = 118.8 MPA **

UTILIZATION STEP1 2.155 / 79.92 = 2.7 %3.62 / 93.2 = 3.9 % *

3.62 / 118.8 = 3.1 % **

STEP 2LOAD SHARING ACC. TO TABLE 2

NEGATIVE WIND LOAD: -2.155 KPA [-45 PSF]

LSF1 = 3.78LSF2 = 1.36

NFL = 11.1 KPAGTF1 = GTF2 = 3.6

LR = 11.1 KPA X 3.6 X 1.36LR = 54.35 KPA

FULL IG MODELSMAX,1 = 1.96 MPASMAX,2 = 5.28 MPA

SALLOW,1 = SALLOW,2 = 93.2 MPA *SALLOW,A,1 = SALLOW,A,2 = 118.8 MPA **

UTILIZATION STEP 2 2.155 / 54.35 = 4 %5.28 / 93.2 = 5.7 % *

5.28 / 118.8 = 4.4 % **

Results

GLASS TYPE STEP OF COMPARISON FAILURE PREDICTION MODEL FPM ALLOWABLE STRESS DESIGN ASD

BIGGER SPAN1500 MM X 3450 MM [59.1 IN. X 135.8 IN.]

6 MM [1/4 IN.] FT & HST20 MM [3/4 IN.] AIR

6 MM [1/4 IN.] FT & HST

STEP 1LOAD SHARING ACCORDING TO

[2]NEGATIVE WIND LOAD:

-2.155 KPA [-45 PSF]

LSF1 = LSF2 = 2.00NFL = 0.89 KPA

GTF1 = GTF2 = 3.6LR = 0.89 KPA X 3.6 X 2.00

LR = 6.41 KPA

LSF1 = LSF2 = 2.00SMAX = 24.83 MPA

SALLOW,1 = SALLOW,2 = 93.2 MPA *SALLOW,A,1 = SALLOW,A,2 = 80.9 MPA **

UTILIZATION STEP 1 2.155 / 6.41 = 33.62 %24.83 / 93.2 = 26.7 % *

24.83 / 80.9 = 30.7 % **

STEP 2LOAD SHARING ACCORDING TO

TABLE 2NEGATIVE WIND LOAD:

-2.155 KPA [-45 PSF]

LSF1 = 2.01LSF2 = 1.99

NFL = 0.89 KPAGTF1 = GTF2 = 3.6

LR = 0.89 KPA X 3.6 X 1.99LR = 6.38 KPA

FULL IG MODELSMAX,1 = 24.85 MPASMAX,2 = 25.13 MPA

SALLOW,1 = SALLOW,2 = 93.2 MPA *SALLOW,A,1 = SALLOW,A,2 = 80.9 MPA **

UTILIZATION STEP 2 2.155 / 7.08 = 30.4 %24.97 / 93.2 = 26.8 % *

24.97 / 80.9 = 30.9 % **

Results

▪ The shown concept and the presented project example shows that a “Allowable Stress

Design” is a useful and suitable practice for finding and verifying a capable glass design as

well as including much more significant boundary conditions than provided by the standard

procedure used in ASTM E1300 [2].

▪ Important considerations for proper design are not just a suitable link between FPM and ASD,

but also a reliable concept of combining different loads with different load durations.

▪ In general, a more accurate concept of load sharing factors is advisable

▪ Essential for proper and sustainable design of IG units is an appropriate concept for

determining climatic effects

Summary

Thank you for your attention !