SUBMITTED BY- VEDANTI BARVEKAR
BOWSTRING TRUSS
What is Bowstring Truss ?
A bowstring truss is a structural device commonly used in bridge-building and, less often, in industrial architecture. Used to span wide, column-free spaces, it consists of an arched beam (the bow) joined at each end by a straight beam (the string), with diagonal support beams joining the two.
oThe first bowstring bridge in America was built over the Erie Canal by Squire Whipple in 1840.
About Bowstring Truss
oThe design worked particularly well for structures that had large, open spaces with no supporting columns in the middle, such as car dealerships and other vehicle storage buildings, supermarkets, bowling alleys and skating rinks.
oAbout 80 years ago, the bowstring truss roof became a popular construction element in the United States.
oThe Bowstring Truss also known as Belfast roof trusses. It is a very efficient structural form with a curved top member that behaves essentially as an arch in combination with a horizontal tie member.
Prior to 1960, it was reportedly one of the most common design types for commercial and industrial structures.
The Whipple Cast and Wrought Iron Bowstring Truss Bridge is located near the entrance to Stevens Farm in southwestern Albany, New York, United States
1. Bowstring trusses can be constructed of wood or steel, but most often, they’re made of wood–often heavy timber.
How Bowstring Truss works ?
2. The principles of bowstring truss construction are similar to other types of truss construction in that web members are used to form multiple series of triangles that transfer tension from the bottom chord and compression from the top chord of the truss onto the load bearing walls.
3. One big difference with the bowstring truss is that the compressional forces within the top chord act to force the load bearing walls outward as well as downward
PROCESS OF CONSTRUCTION
• STEP 1: Lay out the pieces of the arch and fix it together on the ground.
• STEP 2 : Erect the columns.
• STEP 5 : Finally purlins are attached and fixed in position.
• STEP 3 : With the help of cranes, the arches are raised and placed on the top of the columns and fixed.
• STEP 4 : Formwork is used to provide support to the arches.
▪The purlins are located at the trusses and supported curved roof covering.
Details
▪The Belfast roof truss consists of a two-piece bowed top chord and a two-piece horizontal bottom chord.
▪ Lattice (Bracing) members are nailed and notched between the top and bottom chords. The truss profile is very good for uniformly distributed loading.
▪This truss is acted like a tied arch, with the thrust line almost coinciding with the alignment of the top chord, resulting in very small forces in the lattice members.
▪In this Roof trusses, Eaves joints are connected with the top and bottom chords. End Block (eaves) is used to stop the compression force in the top chord and transfer the tension force to the bottom chord.
▪The spacing of purlins is uniform and located along the top chord. The central rise in this type of truss is usually kept about 1/8 the span.
❑ KEY PLAN 60 X 30 M ❑ KEY ELEVATION
SPAN- 30M POST HEIGHT- 6M POST CROSS SECTION – 900 X 900 mm HIGHEST LENGTH OF THE STRUT – 3M
• Detail at A Detail at B
•
❑ JOINERY DETAILS
•Detail at C •Detail at D The gusset plate is placed between the struts and arches and is bolted. Struts- 100 X 300 mm Arches – 200 X 400 mm • Foundation Detail
• Arch in the shape of a parabola is theoretically the most efficient because direct stress alone is developed in the arch and in the tie member.
• Roofs built over Bowstring Trusses have no need to cap the ridge at the top of the roof, saving time and money in the construction process.
• These structures lend themselves to new uses because their spans allow for large skylights and flexible spaces.
• Bowstring trusses are great for spanning large distances- thousands of bowstring trusses were used during World War- II for holding up the curved roofs of aircraft hangars and other military buildings.
ADVANTAGES OF TIMBER BOWSTRING ARCH TRUSS
• A Bowstring Truss may be built to provide the appearance of either a flat or a pitched truss and thus is probably the most flexible of all the truss types.
• Bottom chord fractures and splits can be encountered on Bowstring Trusses.
• Timber is now out-dated and mostly steel Bowstring Arch Trusses are used.
• Buildings with bowstring truss roofs will not only collapse downwards, but can also force the loadbearing walls, upon which the truss sits, outwards in a collapse situation.
• Wood truss members are compromised by water leaks, due to lack of maintenance or by the construction methods used in erecting masonry supporting walls.
DISADVANTAGES OF TIMBER BOWSTRING ARCH TRUSS
EFFECTS OF WEATHER
• The curved top chord allows the buildup of snow and water weight on the roof to cause uneven loading, increasing the stress on the wood members. In turn, the uneven loading can reverse the stress on the wood members causing a “Push-Pull” situation that weakens the wood joints.
• Corrosion on the nails installed on the top chord piles is an indication of prolonged moisture exposure from roof leaks.
• Wood has a high tendency to rot due to presence of moisture.
• Repeated exposure to moisture also affects fastener capacity.
https://www.scribd.com/document/403030348/Bowstring-Truss-Report-docx
https://studylib.net/doc/8911442/the bow string truss jefferson county trails
https://www.dezeen.com/2013/11/26/house conversion with original bowstring trusses works partnership
https://gharpedia.com/blog/belfast roof truss/ https://www.firefighternation.com/firerescue/bowstring-truss-roof-construction-hazards/ https://www.structuremag.org/?p=13262
architecture/#:~:text=A%20bowstring%20truss%20is%20a,support%20beams%20joining%20the%20two.
Reference
https://images.fireengineering.com/wp-content/uploads/2020/09/ancona-bowstring-truss-1.jpg
https://www.flickr.com/photos/artsirois/49947058077/in/photostream/
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