The activities of NEMCON FACADE CONSULT is related to providing professional and technical consultancy services for,
Structural Glazing
CURTAIN WALL is a Generic Term given to a façade of the building that has been covered with any particular type of cladding material - GLASS.
Initially, Glazing were adopted in the form of Windows for the envelope of the building for receiving natural light and to provide ventilation whenever required.
The subsequent architectural aspirations resulted in the development of Curtain Glazing - where the Glazing started getting anchored on the face of the building giving a continuous glazed façade.
In the case of Curtain Glazing, the glass is required to be fixed on to the aluminium framework by means of Mechanical Fixing with External Pressure Plate and Cover Plate.
The architectural aspirations further expected elimination of the band of pressure plate and cover plate being seen from outside in the case of Curtain Glazing.
This resulted in the development of Structural Glazing System where the glass is fixed on to the aluminium frame work by means of structural silicon, thereby eliminating the externally visible Pressure Plate and Cover Plate band.
The Structural Glazing initially was developed with all design and engineering aspects similar to Curtain Glazing with the complete framework of mullions and transoms added with a structurally glazed shutter framework fixed on to the main framework mechanically from the outside of the building.
This modified Curtain Glazing System to Structural Glazing System was termed as STICK SYSTEM BASED STRUCTURAL GLAZING.
The Curtain Glazing and the Stick System Based Structural Glazing are rigidly anchored to the Building Structure with limited capacity to cater to the building end movements.
When the buildings in developed countries were taken up for high-rise structures, lot of engineering efforts were put in to design the structural load, dead load and live load and all material inputs were designed for light weight concept.
Therefore, when the structures were designed with light weight material, excessive permissible design sway and live load movements were expected, resulting in substantial building end movements in vertical as well as horizontal directions.
This further demanded review of the façade glazing system, specially designed to cater to building end movements.
When the buildings were designed high-rise, it was also not possible to carry out the anchoring of the glass to the glazing system from outside of the building.
The subsequent research on these aspects further graduated the glazing system to Unitised System Based Structural Glazing.
The system was thus developed with male-female combination of profiles with pressure equalisation systems to substantially cater to all possible building end movements on the horizontal direction due to permissible design sway of the building as well as on the vertical direction on account of live load.
With the Unitised System Based Structural Glazing, it also became possible to carry out the fixing of the glazed panels on to the structure from the inside of the building.
ARCHITECTURAL ASPECTS
The Architectural Aspect of the Structural Glazing commences from the stage of preparing the elevation drawings for the project.
It has been normally observed that the Architect has a vision of a Glazed Façade for the building, and the available grid is equally divided in vertical as well as horizontal span. In such a situation, it may so happen that the span of a single glass comes partially in the spandrel area and partially in the vision area.
The glass in the Structural Glazing is almost 150 to 250 MM away from the face of the beam. The sill and the soffit is necessary to be sealed. Once the soffit and the sill is sealed, it is compulsorily necessary that the glass should independently span in this area. In case the glass happens to partially span in the vision area and partially in the spandrel area, it would result in thermal variation and breakage of glass.
This aspect of ensuring independent spanning of glass in the spandrel area would change the elevation horizontal lines and this may accordingly have to be related to the surrounding grit wash groove details / stone cladding groove details as well.
There are many such important factors to be taken in to consideration right from the stage of preparing the Elevation Drawings.
FACTORS TO BE CONSIDERED WHILE PREPARING THE ELEVATION DRAWINGS
> Glasses used in the façade glazing are, till date from the overseas source - size limitation - 3210 x 2250mm OR 3210 x 2000mm.
> Panel sizes in the façade glazing must be derived with best optimisation of glass to avoid wastage.
> Designing of wind pressure applicable for the building and façade glazing based on IS-875 (Part-3) 1987 based on key plan dimension factors of overall length, overall width and overall height, mean probable design life of building, location / terrain, category and class of building, topography factors, external pressure coefficient and internal pressure coefficient.
It is necessary that the drawings of façade glazing must have the key plan of the building with dimensions of length, width and height.
> The elevation drawings of the façade glazing must also contain overall glazing dimension, individual panel dimensions horizontally and vertically, all types of typical plans at the glazing area, cross section details with floor to floor dimension, blown up detail of all typical slabs and beams with dimensions of slab thickness, drop of beam, depth of beam, and height of dry wall.
FACTORS TO BE CONSIDERED FOR SELECTING ALUMINIUM PROFILE FOR THE PROJECT
Once the elevation drawings are finalised, the following steps are to be adopted:
> The system of Glazing required for the project - Stick System Based Structural Glazing / Unitised System Based Structural Glazing.
> The Aluminium Profiles will have to be selected after checking its Structural Stability for Deflection and Bending Stress based on the Designed Wind Pressure, Maximum Width of the Panel Size, Distance between Two Supports of Mullion (Floor to Floor Height), Alloy of Proposed Mullion (63400 WP), Modular Elasticity of Alloy 63400 WP (65500 N/MM2 - IS-8147-1976) Permissible Bending Stress - 96 N/MM2 and Permissible Deflection - L/175 for Single Glazing (IS - 8147 - 1976) L/250 for Double Glazing (BS - 8118 - PART ONE - 1991). This way the minimum Moment of Inertia and the Weight of the Profile per Running Metre required for the Load Bearing Mullion for the project can be derived.
Although the design and shape of the profile is derived from the system requirements to suit the concepts of air & water tightness : the structural stability of the profiles is derived from factors mentioned above. Therefore, the same shape and design of the profile can be retained with changes adopted either by increasing the depth dimension or the wall thickness of the profile to suit the structural stability requirements of the individual projects and elevation factors.
This is very important to evaluate the weight of the profiles per running metre for estimation purposes.
FACTORS TO BE CONSIDERED FOR SELECTING GLASS FOR FAÇADE GLAZING
In the case of Commercial High Rise Buildings, lot of public movement is expected at Ground Level around the Curtain Wall / Structural Glazing Area
IN THE EVENT OF BREAKAGE OF GLASS, THE GLASS IN ANNEALED FORM CAN FALL IN LARGE PIECES AND CAUSE CASUALTY.
The glass for Structural Glazing should be a HIGHER SAFETY GLASS to avoid any casualty in the event of breakage of glass.
The first graduation from Annealed Glass to Higher Safety Glass is TEMPERED GLASS.
ADVANTAGE OF TEMEPRED GLASS:
>Only the Impact Bearing Capacity Multiplies and in the event of breakage it falls in splinters.
LIMITATIONS OF TEMPERED GLASS:
>Glass to be ordered cut to size.
>No improvement in Thermal / Acoustic Properties.
>Tempered glass can also break leaving voids in the glazing area.
>Void in the glazing area can become security hazard to building and occupants.
The most suitable glass for the façade glazing is
LAMINATED SAFETY GLASS
The Laminated Architectural Glass is constructed by bonding a tough Polyvinyl Butyral (PVB) interlayer between two pieces of glass under heat and pressure. Once sealed, the glass “Sandwich” behaves as a single unit and appears transparent, just like ordinary glass.
ADVANTAGES OF LAMINATED SAFETY GLASS:
>Although both the glasses being laminated can remain in annealed form, it adds to the impact bearing capacity of the glass and reduces the danger of flying or falling glass in the event of any breakage.
>Available ex-stock and can be cut to required size.
IMPROVED THERMAL PROPERTIES: Screens out 99 Percent of sun’s ultraviolet rays, protecting the interior drapes, carpets, and furnishing from fading and deterioration. It also reduces the thermal losses, thus reducing the consumption of energy for air conditioning.
IMPROVED ACOUSTIC PROPERTIES: The sound isolation glass occurs over the frequency range where glass is almost transparent to sound, due to shear damping characteristics of viscoelastic PVB film.
NO VOIDS: In the event of breakage of the glass, the glass remains in its position and void is not created in the glazing area, thereby allowing the replacement at convenience.
FACTORS TO BE CONSIDERED FOR SELECTING GLASS FOR FAÇADE GLAZING
ADDITIONAL ADVANTAGES OF LAMINATED SAFETY GLASS:
> Protection against forged entries.
> Protection against burglaris.
> Elimination of grills - clear visiblity of glass
> No dangerous splinters or falling glasses
> BULLET RESISTANCE - Most useful for potential target buildings.
> Resistance to intentional or accidental explosion
> Can withstand hurricanes and earth quakes.
> Maintains the integrity of enveloping the building.
> Correct selection of aluminium profile based on the glazing system for the project has to be specified with following details.
Minimum Moment of Inertia Required and Minimum Weight per R.Mt. Of the Load Bearing Mullion Member.
The names of acceptable Primary Aluminium Profile Extruders.
The Surface Treatment required for the aluminium profiles.
Expected / Proposed Profile Drawings.
> Anchoring system to be specified:
Galvanised M.S. / Pure Polyester Powder Coated Aluminium Brackets conforming to relevant standards.
Bi-Metal Corrosion Separators - PVC Spacers between Mullion and Brackets.
Two Nos. of Brackets for each beam - One for Load Bearing and One for Expansion Characteristics of the Mullion.
Aluminium Shims behind brackets / extended brackets to accommodate the beam level variations at anchoring points.
Accepted Size, Type and make of Anchor Fasteners.
> S.S. Quality hardware to be specified:
All Screws, Nuts, Bolts, Washers to be Stainless Steel.
MATERIAL INPUTS
> SEALANTS:
Weather Silicon Sealant type and make for sealing all Expansion Joints at Beam Levels / Transom to Mullion Joints.
Structural Silicon Sealant type, make, application method, design stipulations and curing standards.
> PERIPHERAL PACKING / FLASHING:
Peripheral Aluminium Flashing / Packing and Weather Sealing.
> SPANDREL AREA INSULATION:
Product, type, make and acceptable system of Providing Spandrel Area Insulation.
> SOFFIT LEVEL FLASHING:
PAluminium Sheet, Pure Polyester Powder Coating colour, make, and fixing detail.
> PVB LAMINATED SAFETY GLASS:
Type, Make, Colour (Reflective), PVB Lamination, and Hermatically Sealed Double Glass Units (if required for the Vision Panels).
> ELECTRICITY / SCAFFOLDING / CRADLE ARRANGEMENTS:
The Glazing Contractor to make his own arrangement of Electricity, Scaffolding / Cradle Required for the installation of the Glazing Work.
SPECIFICATION STANDARDS
Structural glazing is specialised and highly sophisticated engineering assignment.
It should not be promoted as a part of main civil tender.
Initial extensive exercise must be carried out related to freezing of the project specifications and drawings.
Specialised agencies must be independently invited to quote for project and based on their individual technical and commercial competence, the project should be awarded.
AMBIGUITY ON COORDINATION BETWEEN THE GLAZING CONTRACTOR AND CIVIL CONTRACTOR:
No Civil Contractor can ensure providing lazer aligned verticality in the masonry or other finishes on the surfaces around the glazing area. This is the reason the Material Input Specification must cover the perimeter sealing / flashing as may be required in the Scope of Work of the Glazing Contractor.
One Standard Condition in the Contract of Civil Contractor:
As long as the Scaffolding of the Civil Contractor is erected in position and not possible to be removed, the glazing contractor or any other contractors expected to carry out their work in the same area, the Civil Contractor will allow the other contractors to use the scaffolding.
SPECIFICATION STANDARDS FOR PRE-QUALIFICATION OF SPECIALISED CONTRACTORS:
> STRINGENT STANDARDS TO BE ADOPTED FOR PRE-QUALIFICATION OF STRUCTURAL GLAZING CONTRACTORS
> TENDERS MUST BE INVITED TO BE SUBMITTED IN THREE SEPARATE ENVELOPES
> THE FIRST ENVELOPE MUST CONTAIN THE EARNEST MONEY DEPOSIT FOR THE PROJECT TENDER.
> THE SECOND ENVELOPE MUST CONTAIN THE COMPLETE TECHNICAL DOCUMENT PRESENTATION FOR THE PROJECT.
> THE THIRD ENVELOPE MUST CONTAIN THE PRICE AND COMMERCIAL BID FOR THE PROJECT.
> IT MUST BE MADE MANDATORY THAT THE FIRST ENVELOPE AND SECOND ENVELOPE SHALL BE OPENED AT THE FIRST INSTANCE. ONLY UPON FINDING THE COMPLETENESS OF THE TECHNICAL PRESENTATION, THE THIRD ENVELOPE OF THE CONTRACTOR SHALL BE OPENED. IN THE EVENT OF NON-COMPLIANCE OF THE TECHNICAL EXPECTATIONS OF THE PROJECT IN THE SECOND ENVELOPE, THE THIRD ENVELOPE SHALL NOT BE OPENED AND THE CONTRACTOR SHALL BE DISQUALIFIED RIGHT AT THAT STAGE.
TECHNICAL SUBMITTALS AT THE TIME OF SUBMISSION OF TENDER - TO BE STIPULATED IN SPECIFICATIONS FOR PRE-QUALIFICATION OF STRUCTURAL GLAZING CONTRACTORS:
> At the time of tendering for the job, the Glazing Contractor must be stipulated to submit the following technical details:
> Designed Wind Pressure Calculations for the project
> Design Calculations to Check the Structural Stability of the Load Bearing Mullion Member for the Un-supported Span (Floor to Floor Height of the Project) - Check to satisfy Permissible Deflection based on L/175 for Single Glass Glazing (based on IS-8147-1976) or L/250 for Double Glass Glazing (based on BS-8118-Part-I-1991) and Check to satisfy Permissible Bending Stress of 96 N/mm2 for Alloy 63400 WP (based on IS-8147-1976).
> Profile Drawings of individual profiles to be used for the project with their dimension, wall thickness and weight Kg./M
> Typical Profile Combination Details for the Project Elevation.
> Drawings of All Types of Brackets required for the Project.
> Calculation of Load on Each Anchoring Fastener in relation to the Manufacturers’ specified Pull Out Load / Capacity.
STRUCTURAL GLAZING IS SPECIALISED AND HIGHLY SOPHISTICATED ENGINEERING ASSIGNMENT
WHILE DRAWING OUT THE SPECIFICATIONS FOR THE STRUCTURAL GLAZING TENDERS, VARIOUS STANDARDS ARE MENTIONED FOR DIFFERENT TYPES OF MATERIAL INPUTS FOR THE PROJECT.
IN MOST OF THE CASES, IT MUST BE CLEARLY SPECIFIED THAT THE MANUFACTURERS’ TEST CERTIFICATES OF THE PRODUCT CONFORMING TO THE RELEVANT SPECIFIED STANDARDS SHALL BE ACCEPTABLE.
HOWEVER, IN CASE THE OWNER DECIDES TO TEST ANY PARTICULAR ASPECT / MATERIAL, THE OWNER SHALL HAVE THE LIBERTY TO GET THE TEST DONE WITHOUT ANY COST LIABILITY TO THE CONTRACTOR.
IN THE EVENT OF FINDING ANY DISCREPANCY BEYOND TOLERANCES IN ANY OF THE PRODUCT USED FOR THE PROJECT, SUCH PRODUCT MANUFACTURERS MUST BE BLACK-LISTED, BUT THE GLAZING CONTRACTOR SHOULD NOT BE PENALISED FOR SUCH DEFECTS OF MANUFACTURING / NON-COMPLIANCE TO STANDARDS.
IN CASE ANY SPECIFIC TEST IS REQUIRED TO BE CARRIED OUT BY THE CONTRACTOR, THOSE TESTS MUST BE CLEARLY SPECIFIED AND THE CONTRACTORS MUST BE ADVISED TO QUOTE THEIR RATES SEPARATELY FOR SUCH TESTS.
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