I have to design a truss bridge for my statics class, and I am interested in finding a truss design with a high strength to weight ratio (the highest wins). The lighter the better, but it needs to be very strong. There re so many truss designs and I am not sure in which direction I should go. any suggestions would help. We are using S4S general purpose pine. It will be loaded in 3-point bending, on two rollers. It must hold at least 2,000 lbs. Thanks!
Generally, a truss which most closely resembles an arch will be the strongest. From the trusses shown here:
I'd recommend the bowstring as the strongest of those. The many members will increase the weight though, so the gambrel would probably be a good choice to lessen the weight yet still retain the strength. That's just a starting point. You can tweak the angles, and take some liberties with the member cross-sections to lessen the weight as well, but the only way to really tell how much is to do an analysis. There are some freeware programs you could use to speed up the process. Here are a few:
Though if you're in school, you probably have access to some better ones.
Another thing to consider is are you really building a "truss" with ideal pinned connections, or is it a "frame?" Most trusses I see are actually frames, because the connections transmit moment, so that's something to keep in mind when doing the analysis.
To get the best strength to weight ratio, try to make it so as many members fail all at the same time. Otherwise, you're overdesigning the members which don't fail.
A flat roof in most homes for sale in Jamaica is essentially a low-pitched roof and is defined as a pitched roof of ten degrees or less to the horizontal. Generally the angle of pitch is governed by the type of finish, which is applied to the roof.
The function of the flat roof is to:
a) Prevent the entry of rain and dust, particularly at joints and junctions
b) Resist the effect of wind (both pressure and suction), sunlight and solar intensity
c) Meet the requirements of current building regulations on structural stability, fire precautions, thermal insulation, condensation, vapor insulation, and loads, dead and imposed, according to whether roof access is permitted or not.
d) Provide a suitable wearing surface
e) Permit movement due to stress and natural causes
Due to Jamaica real estate suffering from the tropical climate many repairs an improvents are needed constantly.
Flat roof construction in timber
The construction of a timber flat roof involves the use of timber joists thirty-eight millimeters to fifty millimeters and from seventy-five to two hundred millimeters deep is placed on edge from four hundred millimeters to six hundred millimeters apart with the ends of the joist built into or against block walls and partitions. The depth of the joist must be sufficient to enable it to span the rooms below without sagging, while the width will normally be fifty millimeters which will allow ample space for fixing boarding.
It should be noted that the strength of the timber joist is not in its width but in its depth, therefore, it is necessary to increase the depth to increase the strength.
The flat roof must be provided with a fall to remove the rainwater as quickly as possible. The direction of the fall will be determined by the position of the roof in relation to other buildings or by its size or shape. This is another critical factor with respect to homes for sale in Jamaica.
To obtain the fall firring pieces are fixed to the top of the roof joists and upon which the roof boarding is nailed.
Firring pieces consist of either tapered lengths of wood nailed to the top of each joist or varying depth lengths of wood nailed across the joists. Tapered firring is used for roofs covered with chipboard and the varying depth firring for boards laid parallel to the slope of the roof so that variations in the level of the boards do not impede the flow of rainwater down the shallow slope.
Flat roof in concrete
Reinforced concrete flat roofs are constructed in the same way as reinforced concrete floors - both in-situ and precast.
Roof Trusses
Trusses are structurally designed frames based on the principles of triangulation. They consist basically of upper and lower chords, tension and compression webs and devices used to fasten the members together.
Trusses are designed to carry roof loads and transmit them to the nearing wall on which they rest. Triangulation of the truss will vary according to span. It is essential that the members of a roof truss are rigidly connected together since light sections are generally used.
Advantages of Roof Trusses
1. Roof trusses save materials and on site labor costs.
2. Roof trusses can also eliminate interior bearing partitions because trusses are self-supporting.
3. Trusses can be erected quickly and therefore the structure can be enclosed in a short time.
4. Root trusses come in a wide variety of shapes to solve nearly any problem.
5. Trusses are usually designed to span from one exterior wall to the other with lengths of six to nine meters or more.
Factors influencing the choice of roof types
•Size and shape of building
•Span
•Appearance
•Initial cost
•Ease with which services can be accommodated in the roof space
•Weather proofing
•Maintenance cost
•Speed of erection
As most designers and construction experts will speak about are the ease with which homes for sale in Jamaica can be improved or repaired.
