This invention is pertaining to the structural stability strength of aluminium mullion interlock
section for sliding windows with height up to 5400 mm against the high wind pressure.
This section profile of mullion is highly suited for areas with high wind pressure and where
sliding windows are in high rising buildings.
2. BACKGROUND
The sliding windows have aluminium mullion interlock where its shutter can be opened through
sliding mechanism. Therefore, the main interlock mullion must be strong enough to provide
stability against the wind pressure. The mullion interlock is checked for:
Bending moment: The measure of bending effect due to forces on a structure and is measured
in terms of force and distance.
Deflection Check: The degree to which a structural element is displaced under load.
The sliding windows are generally with the height up to 3350 mm and it cannot withstand very
high wind pressure of more than 2 kPa. Therefore, mullion has been designed taking in view
the limitation of mullions with greater heights to with stand very high wind pressure (from 1.2
kPa to 5 kPa with feasible window dimensions). The new mullion that has been designed and
referred in this document for patenting has greater MOI, optimum weight and production feasibility.
3. OBJECTIVE OF THE INVENTION
The objective of the invention is to provide high level strength to the sliding window system
with glass through aluminium mullion interlock. The aluminium mullion interlock section provides the strength against the wind pressure to the:
* Sliding Windows located at a very high height.
* Sliding windows with heights up to 5400 mm (17’ 8.5”)
* The aluminium mullion has been designed with a sleek look of 20mm when seen from the
front side of the window.
The structural stability (strength) would depend upon the size of the window and wind pressure.
The mullion has been designed by taking into consideration its geometrical, mechanical
and other properties also.
4. BENEFITS OF THE INVENTION
The major benefits of the invention are as follows:
* Safety: The aluminium mullion interlock is designed to provide safety against heavy wind
pressure and for windows of greater height without supports in between.
* Stability: The aluminium mullion is designed to provide structural stability against heavy wind
pressures.
* Aesthetic: The aluminium mullion was designed by providing the minimal look of 20mm from
the front for better aesthetics and modernized looks.
5. DESCRIPTION
This invention is related to the strength of mullion interlock section for sliding windows to
provide it a structural stability against bending moment and deflection to withstand high
wind pressure.
The mullion interlock is extruded from aluminium alloy specification 6063 T6 and is optimally designed to meet various requirements of functionality and strength. The mechanical and section properties of the profile is as below:
6. DESIGN PROCESS
Required Moment of Inertia
The required Moment of Inertia of the profile to resist the pressure of 1.2kPa for the window
shutter height of 5400mm and width of 2160mm is about 830 cm4
. This value was found after
considering the marginal safety factor of 20% in deflection.
Calculations to derive the required moment of inertia
To derive the required moment of inertia of the profile before its designing for the above-mentioned criteria following formula was used considering the mullion as a simply supported beam
carrying uniformly distributed load:
Required MOI of aluminium interlock mullion section (I) =
5 𝑋 𝑊 𝑋 𝐻^4
384 𝑋 𝐸 𝑋 ∆𝑎𝑙𝑙(𝑤𝑖𝑡ℎ 20% 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑚𝑎𝑟𝑔𝑖𝑛)
Where,
W= Uniformly distributed load on the mullion interlock, KN/m.
L= Length of the mullion interlock profile, mm.
E= Modulus of elasticity of aluminium, mm4.
I= Moment of inertia of the profile, about the bending axis.
∆all= Allowable deflection (with deflection margin of 20%).
Considering the parameters as:
Height of window shutter, H = 5400mm
Width of window shutter, w = 2160mm
Effective module width =w/2 = 1080mm
Wind pressure = 1.2kPa.
UDL on Mullion interlock = 1.296 kN/ m
Modulus of elasticity, E = 70000
I=
5 𝑋 1.296 𝑋 5400^4
384 𝑋 70000 𝑋 24.688
I = 8302996.829/10000
I = 830.29 mm4
Steps to generate aluminium mullion interlock profile geometry
The different aspects which are responsible for the geometry of the profile are as given below
with illustrations:
1. Visible width of the section: The visible width of the section is 20mm, which was fixed to
provide a minimal look and make a better window in terms of aesthetic. (Refer code “A” in
fig: 4)
2. Depth of the section: The total depth of the section is 230mm, which was finalized to
achieve the required moment of inertia of the profile. (Refer code “B” in fig: 4)
3. Glass Infill: The maximum glass infill in the aluminium mullion interlock is of 30mm, as
shown in the illustration. (Refer code “C” in fig: 4)
4. Hollow space: The hollow space provided in the aluminium mullion interlock is to reduce
the overall running weight of the profile and make it economical. (Refer code “D” & “E” in
fig: 4)
5. Space divider: The divider provided in the center of the hollow space of 10mm is to maintain the dimensional stability of the aluminium mullion interlock. So that after hot extrusion
of the profile maintains its dimensional stability. (Refer code “F” in fig: 4)
Fig:5
Weight Calculations
As the cross- sectional area of the profile is As = 1966.3488mm2 and the density of the aluminium alloy 6063-T6 is 2710kg/m3
.
The running meter weight of the aluminium mullion interlock is = 0.00196*1*2710 = 5.32kg
Therefore, the running meter weight of the aluminium mullion interlock is 5.32 kg
7. DIFFERENT SCENARIOS
This section is explained through different scenarios of wind pressure with minimum of
20% marginal deflection within the permissible deflection range of H/175:
Fig: 6
DESIGN SPECIFICATION AND PROCESS
Deflection of mullion
According to ANSI/ AAMA/NWWDA 101/ I.S. 2, for the architectural and heavy commercial hung categories no member shall deflect more than 1/175 of its span.
Therefore, maximum allowable deflection criteria considered for aluminium mullion interlock is H/175, where H is the span of the aluminium mullion interlock.
Fig: 7
SCENARIO -1
Wind pressure: 1.2kPa
Window Parameters
Height of Window (H) = 5400mm
Width of Window (W) = 4320mm
Width of left panel (W1) = 2160mm
Width of right panel (W2) = 2160mm
CHECK FOR BENDING MOMENT
Height of window (H) = 5400 mm
Width of window (W) = 4320 mm
Width of left panel (W1) = 2160 mm
Width of right panel (W2) = 2160 mm
Wind pressure = 1.2 kPa
Wind load on mullion due to wind pressure = 1.296 kN/m
Maximum bending moment on mullion due to (Mb)
=
4.72 kNm
Moment of resistance of mullion profile (Mr) = 7.51 kNm
Margin of safety against Bending Moment (100-
(100*(Mb/ Mr))) =
37.15%
Conclusion: The moment of resistance is greater than the maximum bending moment
with a margin of 37.15%, hence the window is safe.
CHECK FOR DEFLECTION
Actual deflection on aluminium mullion due to
wind pressure ∆act =
24.68 mm
Allowable deflection on aluminium mullion ∆all= 30.85 mm
Margin of safety against deflection (100-
(100*(∆act/∆all))) =
20.00%
Conclusion: As actual deflection is less than allowable deflection with a margin of
20.00%, hence the window is safe.
CHECK FOR BENDING MOMENT
Height of window (H) = 5100 mm
Width of window (W) = 4080 mm
Width of left panel (W1) = 2040 mm
Width of right panel (W2) = 2040 mm
Wind pressure = 1.5 kPa
Wind load on mullion due to wind pressure = 1.53 kN/m
Maximum bending moment on mullion due to (Mb)
=
4.97 kNm
Moment of resistance of mullion profile (Mr) = 7.51 kNm
Margin of safety against Bending Moment (100-
(100*(Mb/ Mr))) =
33.82%
Conclusion: The moment of resistance is greater than the maximum bending moment
with a margin of 33.82%, hence the structure is safe.
CHECK FOR DEFLECTION
Actual deflection on aluminium mullion due to
wind pressure ∆act =
23.18 mm
Allowable deflection on aluminium mullion ∆all= 29.14 mm
Margin of safety against deflection (100-
(100*(∆act/∆all))) =
20.42%
Conclusion: As actual deflection is less than allowable deflection with a margin of
20.42%, hence the structure is safe.
SCENARIO -3
Wind pressure: 2.5kPa
Window Parameters
Height of Window (H) = 4500mm
Width of Window (W) = 3600mm
Width of left panel (W1) = 1800mm
Width of right panel (W2) = 1800mm
CHECK FOR BENDING MOMENT
Height of window (H) = 4500 mm
Width of window (W) = 3600 mm
Width of left panel (W1) = 1800 mm
Width of right panel (W2) = 1800 mm
Wind pressure = 2.5 kPa
Wind load on mullion due to wind pressure = 2.25 kN/m
Maximum bending moment on mullion due to (Mb)
=
5.69 kNm
Moment of resistance of mullion profile (Mr) = 7.51 kNm
Margin of safety against Bending Moment (100-
(100*(Mb/ Mr))) =
24.23%
Conclusion: The moment of resistance is greater than the maximum bending moment
with a margin of 24.23%, hence the structure is safe.
CHECK FOR DEFLECTION
Actual deflection on aluminium mullion due to
wind pressure ∆act =
20.67 mm
Allowable deflection on aluminium mullion ∆all= 25.71 mm
Margin of safety against deflection (100-
(100*(∆act/∆all))) =
19.61%
Conclusion: As actual deflection is less than allowable deflection with a margin of
19.61%, hence the structure is safe.
SCENARIO -4
Wind pressure: 5.0kPa
Window Parameters
Height of Window (H) = 3770mm
Width of Window (W) = 3016mm
Width of left panel (W1) = 1508mm
Width of right panel (W2) = 1508mm
CHECK FOR BENDING MOMENT
Height of window (H) = 3770 mm
Width of window (W) = 3016 mm
Width of left panel (W1) = 1508 mm
Width of right panel (W2) = 1508 mm
Wind pressure = 5.0 kPa
Wind load on mullion due to wind pressure = 3.77 kN/m
Maximum bending moment on mullion due to (Mb)
=
6.69 kNm
Moment of resistance of mullion profile (Mr) = 7.51 kNm
Margin of safety against Bending Moment (100-
(100*(Mb/ Mr))) =
10.91%
Conclusion: The moment of resistance is greater than the maximum bending moment
with a margin of 10.91%, hence the structure is safe.
CHECK FOR DEFLECTION
Actual deflection on aluminium mullion due to
wind pressure ∆act =
17.06 mm
Allowable deflection on aluminium mullion ∆all= 21.54 mm
Margin of safety against deflection (100-
(100*(∆act/∆all))) =
20.80%
Conclusion: As actual deflection is less than allowable deflection with a margin of
20.80%, hence the structure is safe.
8. STRUCTURAL GRAPH
Other then these scenarios the structural stability of the aluminium mullion interlock
section can be determined from the graph given below at 1.2kPa, 1.5kPa, 2.5kPa, 5kPa
for different size of windows. These calculations are done by considering the
minimum marginal safety of factor of 20%.

1. The unique geometrical shape of the designed mullion section with high moment of inertia
the geometrical shape is unique in nature (refer Figure 3 in Design Patent
Specification_1.0.pdf) and no other competitive mullion section of extrusion has the same
geometrical shape and geometrical properties.
2.No other aluminium extrusion is available in the market to withstand high wind pressure
(1.2 kPa to 5.0 kPa) for sliding window with height up to 5400 mm.
3. The sleekness of design (20 mm) from outside provides better look and adequate strength
against high wind load.