4x6 span capacity

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Old 03-03-13, 11:30 AM
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4x6 span capacity

Can a 4x6 cedar beam span 20' without sagging? Building an attached pergola and the beams will carry (essentially) zero load - just a rail system for a fabric shade system.

Second question: is cedar my best choice or is there a better wood type?

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Old 03-03-13, 12:28 PM
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I would doubt that a 4x6 of any species would span 20'-0" without sagging under it's own weight. Of course you would need to determine what you consider an acceptable degree of sagging. Every beam will deflect to some extent and the deflection may or may not have a negative impact on it's ability to do it's job but often times the problem becomes the asthetic issue of looking at a beam that clearly is sagging.

I general as you increase the span distance you need to increase the beam depth. To avoid very deep beams the easiest solution is to decrease the span by adding additional columns or supports of some kind. You could add diagonal kickers at the posts that effectively reduce the clear span of the beam without moving the columns together.

still I believe a 4x6 is not going to go 20'-0" and do what you would like to see it do. Sorry but you may need to rethink your plans.
 
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Old 03-03-13, 04:36 PM
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A 4 x 6 composed of western cedar is woefully inadequate for spanning 20'. It will sag considerably, under just its own weight. Installing a few intermediate supports will make it work, but there are other options if that won't work for you. I'd suggest beefing up the cedar beam with a proper-sized steel angle, installed (with camber up) on the bottom-inside, and attached using appropriate through-bolts. Painted a color to match the stained beam, it would hardly be noticeable to the casual observer.
 
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Old 03-03-13, 06:56 PM
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Thanks. I was afraid of that. The concept of a steel support member might work. Do you have any ideas where I can research them?
 
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Old 03-03-13, 11:28 PM
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Your best approach is to find a local engineer having some practical experience with steel load-strengthening of wooden members. Your local AHJ will require an engineer's stamped plans and calculations before granting a building permit for the structure, so that would be the practical way to get it done, legally. With typical light loading characteristics, it wouldn't require a very stout angle section to span the distance desired without excessive sagging, just by itself. Keep in mind that certain precautions are needed to avoid creating a maintenance or performance nightmare, such as using untreated wood (even though the plan is to use cedar), or allowing water to collect at the interior interface surfaces (promoting rot). In the early 90's, I spent more than $260,000 of DOT money retrofitting 7 glulam pedestrian bridges that had been designed by a firm that didn't think about such things--we applied almost 500 gallons of copper naphthanate preservative, installed many feet of protective shrouding, and field-drilled lots of holes in steel connection fittings to prevent retained water from rotting the wooden members.
 
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