 | New Village Hall |  |
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 I've been asked to write a short piece on the design of your new village hall roof - as Barry has said it does look like a giant jigsaw puzzle with bits of timber everywhere that vaguely resembles the Airfix kits most boys (and some girls!) had a go at building at some point. I will attempt to put it in a lay-persons language but if I drop into "technicalise" please forgive me! Although it may not look like it , most of the hall roof is a standard construction - if you look at the front of the building it is just a straightforward run (run = number of the same type of truss in a line) of mono-pitch trussed rafters (mono-pitch = only slopes in one direction and has a vertical end) with the rafter extended up to the rafters over the meeting rooms/hall and store (these are called flying rafters). Eventually the flying rafters will be supported by pieces of timber between the roof over the meeting rooms/hall/store to make sure they can't dip (these bits of timber are called noggins).  Either side of the main hall at a higher level are two attic area's that have been designed to allow for either stairs or an access ladder up - these are all trussed rafters that Aspect Roofing have made in their factory (as you can see on some of the photos of them being lifted up by the crane). The unusual thing about these attic trussed rafters is that because the roof slope (we call it the pitch) is very steep these trusses actually come in two parts - the bottom bit that has the attic room built into it and a little duo-pitch (duo-pitch = slopes at the same pitch with the highest point central) trussed rafter that sits on top. This is because you have real difficulty transporting anything higher than about 4.0 m (13 foot in old money) on the back of an artic lorry - there is an infamous case in Northern Ireland where someone misread a bridge height and managed to get the lorry and the trussed rafters jammed under a bridge on the M1 out of Belfast and caused the motorway to be shut for six hours in the rush hour! - although the attic area's look complicated they are again a standard construction. Quick bit of digression - a standard trussed rafter works on triangulation - ie all the bits of timber break the internal shape of the trussed rafter into a series of little triangles that structurally are great - this is why on a standard trussed rafter of say 7.0m span (22 foot in old money) you will find all the timbers generally 35mm wide x 72 mm deep. On a attic trussed rafter you lose most of the triangulation (especially where you need it in the middle!) so you will find that the timbers need to be a lot bigger , so for our 7.0 m span trussed rafter as an attic you will find timbers 47 mm wide and ranging in depth from 147 -222 mm deep. Okay - still with me - not bored yet - good - because this is where it gets complicated - THE MAIN HALL (if you have any "Hammer House of Horror sound effects please feel free to add them at this point! | The main hall presented a number of problems - firstly how to support the mono-pitch trussed rafters over the bar and secondly how to support a roof structure 5.0 m (16 foot) in the air. We did this by designing three large timber box beams (box beam = trussed rafter with parallel top and bottom - in this case three big rectangles) that went over the hall to support all the other construction. Because of the large span of these trussed rafters and the load they are carrying these box beams are four trusses all bolted and nailed together on site - again if you look at the photo's of the main hall you can see the two high up in the hall and one over the bar supporting the mono-pitch trusses to the front.  Once these box beams were in position small raised tie mono-pitch trussed rafters were installed either side to form the roof slope to the hall (raised tie = a trussed rafter where the joist/bottom member does not sit on the point of support but is supported by the connection to the rafter) and eventually the top (apex) of this roof will be constructed out of loose rafters. Where this gets further complicated is the fact that a buildings walls work by being buttressed (stopped from bending in/out) by the internal walls or some form of external wall - think about the term "flying buttress" with respect to say Norwich Cathedral. Now in the main hall not only do we have 12 .0 m (38 foot) of unrestrained wall but it is also 3.2 m (10 foot) up , with a big old roof 5.5 m (17 foot) high sitting on top of it - and because of the steep pitch what it wants to do is act as a great big sail! Now it is all very well fixing all the bits of the building together properly , but if we don't restrain the walls and roof from moving when the wind blows very soon half of your village hall will end up blowing into the North Sea!
We worked with the consulting engineer extensively on this (big shout out to him by the way) and designed two wind beams (basically a box beam laid flat) with all the fixings of the walls to the raised tie mono's - raised tie mono's to the box beams (still with me) and box beams to the wind girders to take all the wind loads onto the building into the wind beams which are then secured to the walls at either end of the hall which have some mass concrete at the top to put the loads into the walls and then foundations ! (I did warn you it got complicated !). If you look at the photo's over the main hall you can see the two box beams standing vertically with one wind girder between them and one wind girder on top of them. Once this had all been figured out it was just a case of presenting it in a form people could understand without the need of alcohol - which is where we come all the way back to our Airfix kit at the top of this - with an honourable mention to the many hours I spend building Lego with my 10 year old boy! Thanks for bearing with me and reading this - don't worry - there won't be a test on the industry jargon! |
