Truss Designs
Truss Designs Trusses For ultimate strength and to fit within the client’s criteria, a truss design will be used for the observation deck. Not only does a trussed cantilever better strength than a suspension design it also ensures that there are no obstructions to the view above the deck. This will give sightseers 360-degree views of the breathtaking scenery around them. Truss girders are an efficient and economical structural system where the material is fully utilised. For short to medium spans, it is more economical to use a parallel chord truss such as a Warren, Pratt or Howe truss as these designs minimise the cost of fabrication and erection. However for short spans, a Warren truss is more appropriate as it is more economical as it uses fewer materials than a Howe or Pratt truss. For aesthetic purposes, it is ideal to have an even number of bays on a Pratt or Warren design to avoid a central bay with cross diagonals.
Compressive Members A compressive member, or strut, is a component of a structure that undergoes “pushing” forces when under load. This “pushing” or “squeezing” force is constant throughout the member; both ends experience the same force. When designing a compressive member on a truss, some knowledge about how different members act when under a load needs to be understood to get the ultimate strength out of the design. When a load is placed onto a compressive member, it is stronger in one plane compared to another and is just as likely to buckle in plane or out of plane. To account for this, a compressive member should be designed to be as short as possible with additional bracing considered to prevent any buckling.
Tension Members A tension member is a component of a structure that undergoes “pulling” forces when under load. Just like a compressive member, this force is constant throughout the member with both ends experience the same force. These members should be as compact as possible to allow for the “stretching” forces that they undergo. A tension member should be kept as light as possible while still be structurally strong to keep overall costs down for the structure.
Warren Truss Patented by James Warren in 1848, the Warren truss is the most popular and recognisable truss bridge design in the world. This design uses equilateral triangles to spread loads, minimising forces to only compression and tension. As a load moves across a Warren truss bridge, centre members can swap between compression and tension depending on the position of the load. With minimal
components, this design requires fewer materials than others, making it the most economical for short spans.
Materials Balsa Wood When it comes to light, strong materials for constructing the scaled model, there is no better economically viable material like Balsa Wood. Being quite a dense timber, Balsa is the only timber in the world that is both a hard wood and a soft wood due to its properties. With a range of densities on the market, it can be quite hard to determine the right type of Balsa for the model. The denser the sheet, the more it will weigh however this relationship between weight and strength is not always constant. Looking at Table 1, the tensile and compressive strength of different densities of Balsa can be compared to determine the right grade for each member in the truss. Table 1 Compressive Strength* Low Density Medium Density High Density Tensile Strength* Low Density Medium Density High Density Elastic Modulus – Compression Elastic Modulus - Tension *Low Density: 75 kg/m3
Medium Density: 150 kg/m3
4.7 MPa 21.1 MPa 19.5 MPa 7.6 MPa 19.9 MPa 32.2 MPa 460 ± 71 MPa 1280 ± 450 MPa High Density: 225 kg/m3
Balsa is produced in two distinct cuts and everything in between, “A”, “B” and “C” grains. Strips of “A” grain are tangent cuts to the growth rings. This means that the growth ring apex is in the strip of timber making it very flexible but lacking stiffness. Pieces of “C” grain balsa are cut perpendicular to the growth rings, this makes the strips quite stiff but brittle. “B” grain balsa are random cuts from the balsa tree, every sheet varies in properties. A single sheet of “B” grain may vary from “A” to “C” across its width.
Polyvinyl Acetate (PVA) Glue The most common timber glue, Polyvinyl Acetate, typically known as PVA Glue, is a thermoplastic polymer composed of polymers of vinyl acetate. It is usually preferred due to its range of use and its ability to be used with bare hands. As PVA glue does not emit any harmful fumes, it is also one of the safer options for bonding two surfaces together. When used with timber, PVA glue soaks into the grains of both timbers, usually making the bond stronger than the timber itself.
Bibliography
Design of Steel Structures, Prof. S.R. Satish Kumar and Prof. A.R. Santha Kumar Oxford Dictionaries (copyright © 2012) oxforddictionaries.com/definition/truss?q=truss oxforddictionaries.com/definition/strut?q=strut Garrett Boon (4/1/2011), Warren Truss, (13/4/2012) www.garrettsbridges.com/design/warren-truss/ Garrett Boon (4/1/2011), Pratt Truss, (13/4/2012) www.garrettsbridges.com/design/pratt-truss/ Balsa Wood Properties Guide (pdf) accessed at: www.auszac.com/Balsa%20wood%20Properties%20Guide.pdf Paul L Daniels, Working with Balsawood: Attributes of Balsa Wood, (21/4/2012) pldaniels.com/flying/balsa/balsa-properties.html G.T. Murray, 1997, CRC Press, Handbook of Materials Selection For Engineering Application PVA Glue, PVA Wood Glue, (21/4/2012) pvaglue.net/pva-wood-glue/
Compressive Members A compressive member, or strut, is a component of a structure that undergoes “pushing” forces when under load. This “pushing” or “squeezing” force is constant throughout the member; both ends experience the same force. When designing a compressive member on a truss, some knowledge about how different members act when under a load needs to be understood to get the ultimate strength out of the design. When a load is placed onto a compressive member, it is stronger in one plane compared to another and is just as likely to buckle in plane or out of plane. To account for this, a compressive member should be designed to be as short as possible with additional bracing considered to prevent any buckling.
Tension Members A tension member is a component of a structure that undergoes “pulling” forces when under load. Just like a compressive member, this force is constant throughout the member with both ends experience the same force. These members should be as compact as possible to allow for the “stretching” forces that they undergo. A tension member should be kept as light as possible while still be structurally strong to keep overall costs down for the structure.
Warren Truss Patented by James Warren in 1848, the Warren truss is the most popular and recognisable truss bridge design in the world. This design uses equilateral triangles to spread loads, minimising forces to only compression and tension. As a load moves across a Warren truss bridge, centre members can swap between compression and tension depending on the position of the load. With minimal
components, this design requires fewer materials than others, making it the most economical for short spans.
Materials Balsa Wood When it comes to light, strong materials for constructing the scaled model, there is no better economically viable material like Balsa Wood. Being quite a dense timber, Balsa is the only timber in the world that is both a hard wood and a soft wood due to its properties. With a range of densities on the market, it can be quite hard to determine the right type of Balsa for the model. The denser the sheet, the more it will weigh however this relationship between weight and strength is not always constant. Looking at Table 1, the tensile and compressive strength of different densities of Balsa can be compared to determine the right grade for each member in the truss. Table 1 Compressive Strength* Low Density Medium Density High Density Tensile Strength* Low Density Medium Density High Density Elastic Modulus – Compression Elastic Modulus - Tension *Low Density: 75 kg/m3
Medium Density: 150 kg/m3
4.7 MPa 21.1 MPa 19.5 MPa 7.6 MPa 19.9 MPa 32.2 MPa 460 ± 71 MPa 1280 ± 450 MPa High Density: 225 kg/m3
Balsa is produced in two distinct cuts and everything in between, “A”, “B” and “C” grains. Strips of “A” grain are tangent cuts to the growth rings. This means that the growth ring apex is in the strip of timber making it very flexible but lacking stiffness. Pieces of “C” grain balsa are cut perpendicular to the growth rings, this makes the strips quite stiff but brittle. “B” grain balsa are random cuts from the balsa tree, every sheet varies in properties. A single sheet of “B” grain may vary from “A” to “C” across its width.
Polyvinyl Acetate (PVA) Glue The most common timber glue, Polyvinyl Acetate, typically known as PVA Glue, is a thermoplastic polymer composed of polymers of vinyl acetate. It is usually preferred due to its range of use and its ability to be used with bare hands. As PVA glue does not emit any harmful fumes, it is also one of the safer options for bonding two surfaces together. When used with timber, PVA glue soaks into the grains of both timbers, usually making the bond stronger than the timber itself.
Bibliography
Design of Steel Structures, Prof. S.R. Satish Kumar and Prof. A.R. Santha Kumar Oxford Dictionaries (copyright © 2012) oxforddictionaries.com/definition/truss?q=truss oxforddictionaries.com/definition/strut?q=strut Garrett Boon (4/1/2011), Warren Truss, (13/4/2012) www.garrettsbridges.com/design/warren-truss/ Garrett Boon (4/1/2011), Pratt Truss, (13/4/2012) www.garrettsbridges.com/design/pratt-truss/ Balsa Wood Properties Guide (pdf) accessed at: www.auszac.com/Balsa%20wood%20Properties%20Guide.pdf Paul L Daniels, Working with Balsawood: Attributes of Balsa Wood, (21/4/2012) pldaniels.com/flying/balsa/balsa-properties.html G.T. Murray, 1997, CRC Press, Handbook of Materials Selection For Engineering Application PVA Glue, PVA Wood Glue, (21/4/2012) pvaglue.net/pva-wood-glue/
