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Cob: Berkeley living roof & wall loading vs. earthquakesben graham benfrankg at hotmail.comSun Feb 11 18:51:33 CST 2001
<html><DIV></DIV> <P>I am inclined to look more closely first at the reasons for designing cob as infill which inherently does not make sense. I understand the reasoning, of wanting to build the roof first, which does make sense. There <U>are </U>ways of lowering the roof off temp posts onto the builtup, bearing cob walls. Regarless, if we find ourselves with a cob infill system what are the forces?</P> <P> From all the figuring that Ive seen of forces acting on a loosely integrated system(infill)(debatable depending on specific design) and a monolithic bearing wall, the monolith wall will usually be more stable. I also understand that when we are talking about EQ's there is a degree of play that is pretty loose, considering the wide spectrum of forces that each EQ might give to a structure. And most codes work with the understanding of looking for a system that will not fail catastrophically. With this idea, I believe the degree of difference between an infill wall and a bearing wall is lessened. If we look at ways to strengthen the infill wall to out of plane forces, which is becoming more theoretical and less exact given the true nature of EQ forces, ideas like installing vertical reinforcement comes to mind. This means doing more to an already laborious system that is questionable in the first place.</P> <P> My feelings tell me to rethink the value of designing an infill wall using cob. If the sole reason is to be able to build the roof first, then Im not sure it has enough logic to convince me considering the inherent value of cob and the work it takes to construct given other options. </P> <P>Regards,</P> <P>Ben Graham</P> <P>>Hi all cobbers, </P> <DIV></DIV>> <DIV></DIV>>Shabari Bird & I had this brief communication last month & I'm finally <DIV></DIV>>getting around to bringing it to the cob list." <DIV></DIV>>This is intended to discuss the issue of cob walls being constructed as <DIV></DIV>>either "load bearing" or "infill". <DIV></DIV>> <DIV></DIV>>The exchange (in reverse order) went: <DIV></DIV>> <DIV></DIV>>Shabari Bird wrote: <DIV></DIV>> > <DIV></DIV>> > I would like to have this discussion on the list <DIV></DIV>>.......... <DIV></DIV>>John Fordice wrote: <DIV></DIV>> > > Shabari, <DIV></DIV>> > > <DIV></DIV>> > > Question: Where are you located? I ask because of concern for the roof <DIV></DIV>> > > first approach it sounds like you are using. An ok technique in non <DIV></DIV>> > > earthquake areas, but a bit suspect in my opinion if you are located in <DIV></DIV>> > > EQ county. Testing/observation has shown that earthen walls which are <DIV></DIV>> > > carrying an evenly distributed roof load will survive an earthquake <DIV></DIV>> > > better than an infill earth wall system in a post and beam structure. <DIV></DIV>> > > Am I correct in the latter is what you are doing ? <DIV></DIV>> > > <DIV></DIV>> > > I would like to move this discussion to the open list if it's ok with <DIV></DIV>> > > you, as it's an important issue that I'd like to see others join in to. <DIV></DIV>>.................. <DIV></DIV>> > > <DIV></DIV>> > > > Shabari Bird wrote: <DIV></DIV>> > > > <DIV></DIV>> > > > We are currently installing a living roof. We will be cobbing the <DIV></DIV>> > > > room in August. OUr builder is building the roof this coming week. <DIV></DIV>> > > > The room is 35 feet by 13 feet. We have the liner left over from our <DIV></DIV>> > > > pond installation. We plan to put bales of straw and hay on top of <DIV></DIV>> > > > the liner instead of struggling with getting large amounts of soil up <DIV></DIV>> > > > to the top,. <DIV></DIV>> > > > We will then wait until nature makes the soil and then we plant with <DIV></DIV>> > > > creeping rosemary. <DIV></DIV>>.................... <DIV></DIV>> <DIV></DIV>>So, as I've stated above, the sometimes adopted technique of building <DIV></DIV>>cob as an infill in a post and beam structure has a basic structural <DIV></DIV>>problem to it. <DIV></DIV>> <DIV></DIV>>Post and beam structures are by their very nature, a series of straight <DIV></DIV>>walls with the loads of the roof being carried by the beams to the posts <DIV></DIV>>and thence to the ground. This eliminates the roof load from going into <DIV></DIV>>the cob walls. A reasonable technique to structurally satisfy a building <DIV></DIV>>department that does not understand cob, or to speed up a cob building <DIV></DIV>>while protecting the cobbing by placing it under an already standing roof. <DIV></DIV>>The rub comes in the nature of the inherent strength of cob, and how <DIV></DIV>>earthquakes (EQ) act on straight walls. Cob is very strong in <DIV></DIV>>compression. One way earthquakes act on a wall is what is called "in <DIV></DIV>>plane". This is when the EQ forces push & pull in line with the length <DIV></DIV>>of the wall & in that direction there is lots of cob to resist the <DIV></DIV>>forces and alot of these forces can be taken up in compression of the cob. <DIV></DIV>> <DIV></DIV>>The other way EQs act on a wall is known as "out of plane", or sideways <DIV></DIV>>to the wall. In this case, there is very little wall cob to resist the <DIV></DIV>>EQ forces, and the perpendicular direction of the load essentially puts <DIV></DIV>>the forces into the cob as tension, and tension is where cob is weakest. <DIV></DIV>> <DIV></DIV>>ALWAYS REMEMBER, COB IS STRONG IN COMPRESSION AND WEAK IN TENSION. <DIV></DIV>> <DIV></DIV>>The advantage of loading a cob wall at the top is that you will create a <DIV></DIV>>system of restrain for the "out of plane " forces at the top of the <DIV></DIV>>wall, and should the wall fracture from the forces of the EQ, you <DIV></DIV>>additionally will have gravity loaq forces pushing down on the wall <DIV></DIV>>which will help keep the fractured portions of the wall in place. It is <DIV></DIV>>best if the roof loading is as uniform as possible (cob doesn't like <DIV></DIV>>highly concentrated loads) and will be further helped of you have some <DIV></DIV>>sort of a "bond beam" atop the wall which serves to grip the top of the <DIV></DIV>>wall and resist the "out of plane" forces at the top of the wall while <DIV></DIV>>at the same time serving as a mechanism to evenly distribute the many <DIV></DIV>>point loads created by the roof rafters where they bear on the wall. <DIV></DIV>> <DIV></DIV>>This is not just pie in the sky theory. Observation of failure of earth <DIV></DIV>>walls due to EQ forces as shown this to be true, and the system of a <DIV></DIV>>wall top bond beam which is well attached to the wall AND to a strong <DIV></DIV>>roof diaphragm has been tested on adobe walled structures an proven to <DIV></DIV>>resist strong EQ forces. This added to the apparent( though as yet <DIV></DIV>>unproved) superior strength of cob over adobe will impart a substantial <DIV></DIV>>level of ER resistance to a well built cob structure. <DIV></DIV>> <DIV></DIV>>I welcome comments & reactions to this. <DIV></DIV>> <DIV></DIV>>Re the Berkeley living roof: The EPDM pond liner roof membrane went up <DIV></DIV>>pretty much without a hitch two Saturdays ago. The one problem was that <DIV></DIV>>due to the slope of the conic section form of the roof, the EPDM tended <DIV></DIV>>to slip down the roof & create a bit of tension. We solved this <DIV></DIV>>hopefully by attaching it temporarily at the top & then releasing it & <DIV></DIV>>reattaching once we had the soil in place. In hind sight, it would have <DIV></DIV>>been a good thing to include some sort of adhesive attachment of the <DIV></DIV>>underside of the EPDM to the roof sheathing at least down the center of <DIV></DIV>>the roof slope. Next time. <DIV></DIV>> <DIV></DIV>>The light weight soil mixture as placed the next day oh Sunday with the <DIV></DIV>>help of several folks who responded to the Cob list & ADPSR email call <DIV></DIV>>for help. Thanks to those who came to help, we were able to make a <DIV></DIV>>bucked brigade & place the 5 cubic yards of soil in about one hour. We <DIV></DIV>>did it as two layers of appx 3" with a layer of plastic bird netting in <DIV></DIV>>between the two layers to serve to tie it all together on the compound <DIV></DIV>>sloping roof. Once the native grasses & wild flowers take root, we <DIV></DIV>>should have a well tied together and stable layer of soil on the roof. <DIV></DIV>>The soil mix and selection of grass & flower seeds was done by Heather <DIV></DIV>>Koch. <DIV></DIV>> <DIV></DIV>>Thanks to all who made this milestone happen. <DIV></DIV>> <DIV></DIV>>The recycled plastic bender board fascia is all that's left to be done & <DIV></DIV>>the the roof will be complete. The building should be finished in early <DIV></DIV>>March. It's been a long 2 1/2 years of Saturday sessions getting there, <DIV></DIV>>but well worth the effort. It's a beautiful little building. <DIV></DIV>> <DIV></DIV>>john fordice <DIV></DIV>>maker of cobbers thumbs <DIV></DIV>>the cod code project <DIV></DIV>> <DIV></DIV><br clear=all><hr>Get your FREE download of MSN Explorer at <a href="http://explorer.msn.com">http://explorer.msn.com</a><br></p></html>
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