View source for Design, Make and Test a Warren Girder


===== What to Make: =====
Design Make and Test a [http://en.wikipedia.org/wiki/Girder '''Warren Girder'''] truss frame 450mm wide to span a 400mm wide gap and support a centrally applied load of 1kg


[[File:BrinningtonBridge.png |400px|right|link=http://www.ipernity.com/doc/302581/30372307]]
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=====What to Use:=====

* Material available is restricted to 3mm thick medium density [http://en.wikipedia.org/wiki/Ochroma_pyramidale '''Balsa Wood'''].
* Use any suitable '''[[Adhesive Types|adhesive]]''' to make connections but '''[[Adhesive Tapes|adhesive tape]]''' is not allowed.


=====Things to find out and think about:=====

* Draw full-size a Warren Girder exactly 450mm wide
* Analyse the forces acting within a Warren Girder and determine which '''[[Structural Member|Members]]''' will be '''[[Strut|Struts]]''' and which will be '''[[Tie|Ties]]'''.
* Apply '''[[Bow's Notation]]''' and use '''[[Force Diagrams]]''' to determine the '''[[Forces in Frames|Forces in the Frame]]''' when the 1kg load is applied.
* Determine the '''[[Tensile Strength]]''' of medium density Balsa Wood either by testing or looking up in tables and from this establish how wide to cut '''[[Tie|Ties]]''' from 3mm thick sheet Balsa ''(e.g. U.T.S. should be around 15Nmm<sup>2</sup> - use this figure if in doubt or to check magnitude of test results)''.
* Apply '''[[Buckling|Euler's Buckling Equation]]''' to inform an estimate of how wide to cut the '''[[Strut|Struts]]''' from 3mm thick sheet Balsa ''(e.g. assume a figure for '''[[Young's Modulus]]''' (E) of 4000Nmm<sup>2</sup> or determine by experiment by measuring the '''[[Beam Deflection|Deflection]]''' of a balsa sample beam for example)''.



=====Some ideas:=====

[[File:FramePlan.png|400px]][[File:WarrenFrame.png|400px]]


=====Ways of Making:=====


* Conduct some tests to establish which is the most effective adhesive and consider how each joint can be provided with an adequate gluing surface area.
* Take care when shaping and finishing the '''[[Structural Member|Structural Members]]''' to avoid any ''nicks'' or sharp indentations which might cause a '''[[Stress Concentration]]'''.
* Use the full-size drawing to position the '''[[Structural Member|Structural Members]]''' accurately and glue them in place.
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<span style="color: green">'''Note:'''
Cover the drawing with tracing paper to enable the truss frame to release after gluing. 
</span>
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[[File:FrameTestRig.png|400px|right]]
=====Testing:=====

* Carefully weigh the completed truss frame ''(ideally to 1/10<sup>th</sup> of a gram)''.
* Mount the completed truss frame in the testing rig as shown
* Suspend a mass set hanger from the centre as shown
* Add masses carefully to avoid generating a '''[[Live Load|Dynamic Load]]''' until the specified load of 1kg is reached.
* Divide 1kg by the truss frame self-weight to gain a measure of the frame's efficiency ''(i.e. high numbers win!)''



=====Further Work:=====
 

* Continue to load the frame until it fails and record the maximum load before failure.
* Divide the maximum load before failure by self-weight to gain a revised measure of the frame's efficiency.
* Observe how the frame failed : which '''[[Structural Member|Member]]''' failed first? Was it a '''[[Strut]]''' or a '''[[Tie]]'''? Did just one '''[[Structural Member|Member]]''' fail or several all at once - and what would this indicate? 


{{Stripwood Frames Buyers Guide}}


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[[Category:Structures Projects]]
[[Category:Bridges]]