Glass Foot Bridges - Case Study Example

Glass Foot Bridges Glass has become the new leader in bridge construction. It is supposed to be more long lasting than the concrete bridges. Concrete bridges corrode faster because of the iron rods. Glass fibers can extend the life of a bridge from 50 years to 200 years says John Scalzi, who directs the National Science Foundation's (NSF) large structural and building system program. Glass has multifunctional potential in high technology engineering and can be used for structural applications. Conventionally it is used due to its property of isolation and transparency. Glass is being considered as a strong contender for building materials. Apart from isolation and transparency, it is also very durable. Some other mechanical properties that make it an attractive option in engineering applications are strength, stiffness, chemical resistance and fire protection. For using glass in a foot bridge, we need to know about the stability, strength and production of structural glass. Glassy structures are found in a broad range of solids like polymer, dipolar, and bio-molecular networks. Magnetic materials and granular media also have glassy structures. The mechanical properties of structural glasses do not have too many topological defects like dislocations. Quantum effects take over at low temperatures and glasses show universal properties at low temperature and their dynamics change as much as 1 mK. There are different methods for making glass, resulting in different types of glass. There are many sub-types and categories from basic window glass or float glass to brightly colored stained glass. Apart from this, there are different glass coatings like dichromic and iridescent. These coatings have unique properties when they are applied to glass. Different glass types can be fused, slumped or can be put in other kiln-forming processes. We need to understand compatibility while selecting a glass type. The change in density due to change in temperature, which occurs at the molecular level is known as the Coefficient of Expansion (COE). These differences in expansion and contraction are very significant while deciding the compatibility. If the COEs of two glasses are very different, they will be incompatible. For structural purposes, mostly laminated and toughened glass types are considered. Laminated glass has an interlayer of PVB or polyvinyl butyral between its layers that can be two or more. This holds the glass in place if it is shattered. Due to the interlayer the layers of the glass remain bonded even when they are broken. They do not break into sharp large pieces due to its high strength. If the impact is not high enough to break the glass, it cracks into a spider web pattern. Due to these properties, laminated glass is used for structural purposes when a human impact is possible or falling of glass is possible if it is shattered. While choosing material for a glass footbridge, both these possibilities are there. Another advantage it has as a structural material is its sound insulation property. Due to the PVB interlayer, it has a high sound insulation and also 99% blockage of UV rays. Another type of glass normally used for structural purposes is the toughened glass or the tempered glass. It has increased strength and breaks into small cuboid pieces when broken. It is generally used as the top of the pinball machine. In buildings, it is used as frameless doors and other places where there is a possibility of human impact that could be dangerous. Most of the time a glass breaks if the damage occurs near the edge of the glass as that area has highest tensile stress. Toughened glass can shatter if it is hit hard intentionally in the middle. This is a security threat if it is to be used for structural purposes. A relatively new construction material is Recycled glass aggregate. It has includes 100% glass and glass-aggregate mixture. It is easy to place, durable, strong, and compact. One needs specifications regarding the cullet gradation, cullet content, compaction level, and debris level before putting it to use. These specifications should be about the engineering behavior of the in-place material so that it can be properly used. Glass aggregate in normally used in load-supporting applications. Load-supporting backfill supports heavy stationary loads like slabs, footings or pedestrian sidewalks. These fills must be strong and there should be minimal settlement potential under applied loads and material’s weight. For getting the required density, the material can be compacted. By controlling the gradation and deleterious debris content the settlement potential can be reduced. Glass aggregate being a granular material deforms elastically under load, but when the load is removed it returns to its original volume. A picture of a glass footbridge Non structural issues Lighting is an important aspect due to the public nature of the bridge. Both the day lighting conditions and night lighting conditions mist be considered. One must take into account the effect of the shadows of surrounding buildings and also the thermal comfort of the space if it is going to be an enclosed bridge. The climatic conditions of the place should also be considered while designing the bridge. During the day there is glare and photo-chromatic glass can be used to remove that. If the bridge gets the shadows of other surrounding buildings, then also photo-chromic glass that has the ability to darken under the varying sun conditions of the location, is appropriate. if we use elechtrochromic system with a polarizing filter, the bridge can get a regulated interior lighting Because the bridge needs to be lighted for twenty-four hours, proper lighting should be provided by simple incandescent lamps which needs higher maintenance as well as a higher energy. Fluorescent strip lamps have a reduced operating cost, but look more artificial. Depending on the aesthetic needs, this choice can be made.. Because of the public nature of the bridge a full air conditioning system will be needed. The transparency may need to be reduced in order to reduce glare. This can be done either by putting a film on it or using a less glazed glass. Since a bridge is an outdoor passage, the glazing should be reactive to the ever-changing lighting. For this photo chromatic glass is the best choice. It would also react well to the shadows arising in midday. There are three possibilities :- a. Photochromic glass which can darken quickly when exposed to excessive sunlight b. Electrochromis glass: normally seen in movies, the glass becomes opaque when one turns on a switch. This technique can also be used to and increase shading and reduce transparency and darken the glass. c. Glass with polarized filters: These filters take away the glare resulting from excess light. Daylight analysis/image night light anlysis/image Glass surfacing can be used for footbridges. These plates can be 1800mm wide and 900mm wide. The top layer can be formed with a 8mm tempered glass. Top layer can be laminated on 4 layers of 10mm thick laminated glass sheets. Total it will be 51.8 mm thick. To provide the opacity some of the layers should be coloured. These have to be supported by a 5mm by 80mm neoprene strip. It gives longitudinal support to the panels in the longitudinal direction. A neutral silicon band needs to be used between the joints pf the glass panels. For making it skid resistant, surface treatment should be provided in 20mm bands that can be spaced at 40mm longitudinally. Care has to be taken about the drainage as it is different from the road bridges or rail bridges. We can not put drainage gullies like we put on road and rail bridges. It is better to place the drainage in the middle part of the foot bridge as it will look more elegant. Shadow analysis forms an important part in case of a glass foot bridge. A graph needs to be drawn of the shadows at different times of the day. Accordingly thermal and lighting decisions can be taken. Air conditioning system will also be decided according to the shadow analysis. Air conditioning any interior is different from a foot bridge as a foot bridge is exposed to outer radiation and thermal conditions. There are certain very good examples of glass foot bridge. The Chihuly Bridge is one of them.The Chihuly Bridge of Glass is a 500-foot-long pedestrian bridge linking downtown Tacoma, Washington, to the city's waterfront, the Thea Foss Waterway. It was conceived by Dale Chihuly and designed in collaboration with Arthur Andersson of Andersson·Wise Architects. it is a display of color and form soaring seventy feet into the air. The Chihuly Bridge of Glass, is a piece of structure as well as a piece of art. It also required proper planning as most other bridges do. Design of footbridge has the same principles as other bridges. Since they are normally very lighter than a vehicular bridge, they can be more vulnerable to vibration. One must keep in mind the dynamic effects in its design. Foot bridges in Paris (Pont de Solferino) and in London ( Millenium Bridges) had faced such problems recently. Structural design of the foot bridge needs to keep in mind the vibration analysis of the bridge. Pedestrian footbridges face this problem when the structural fundamental frequency is near the load excitation frequencies, or higher frequencies multiples according to Silva et al. That is why a linear elastic finite element analysis must be done. So , while choosing the right type of glass is important ,other non structural issues also have as important a role for the practicality of the footbridge. Reference: Tredway, W. K. ; Prewo, K. M., UNITED TECHNOLOGIES RESEARCH CENTER EAST HARTFORD CT, Carbon Fiber Reinforced Glass Matrix Composites for Structural Space Based Applications. 31 JUL 1989  Glass Bridges Stronger than steel, these dazzling structures will change the art of bridge building. BY JIM WILSON Published in the December 1997 issue. Costa, S., Miranda M., Varum H. and Teixeira-Dias F., 2006, On the evolution of the mechanical behaviour of structural glass elements, Materials science forum, vol 514-516, pp709-833 Schlaich, Mike, et al., Guidelines for the Design of Footbridges, International Federation for Structural Concrete, 2005, Schlaich, Mike, Source: International Journal of Space Structures, Volume 22, Number 1, March 2007 Mechanical behaviour of Glassy materials, UBC, Vancouver, Canada, July 21-23, 2007. http://pitp.physics.ubc.ca/confs/glass07/ The Behavior of Glass Aggregate Under Structural Loads Developing Specifications for Waste Glass and Waste-to-Energy Bottom Ash as Highway Fill Materials, Volume 2 of 2 (Waste Glass), Paul J. Cosentino Ph.D., P.E., et al., Florida Institute of Technology, 1995. Case Studies for the Use of Post Consumer Glass as a Construction Aggregate, CWC report GL97-5rpt, 1997. Issue Date / Update: November 1996 http://www.chihuly.com/bridgeofglass/projectdescription.html Appendices SUMMARY OF COEFFICIENT OF EXPANSION  FOR COMMON GLASSES AND METALS (with melting points for common metals) Glass information Metals information Type of Glass Coefficient of expansion Bullseye tested compatible (Also Uroboros 90) 90 Effetre (Morretti) sheets and rods (some variation; should test) 104 Spectrum System 96 (also Uroboros 96) 96 Borosilicate (Pyrex) 32.5 Window (float) glass (Also includes most bottles)  83 to 87 (depends on manufacturer) May be even higher or lower Source:  Manufacturer's data   Type of metal Coefficient of Expansion Melting point (°F) Melting point (°C) Aluminum 248 1218 659 Brass, navy 212 1650 900 Copper 176 1981 1081 Gold 140 1945 1061 Iron, cast 108 2300 1260 Lead 295 621 328 Silver 191 1764 962 Steel, high carbon 121 2500 1374 Steel, stainless 171 2600-2750 1430-1507 Tin 398 788 415 Note:  These are for pure metals.  Alloys can vary widely.  I have seen other sources with slightly different COEs, but most are close to these figures.  (And besides, they're close enough for government work.) Source:  U.S. Military Training Circular No. 9-237, "Welding Theory and Application."   Read More