Replies

1. 
   Dan_Metzcus | Dec 13, 2000 09:29am | #1

   *
   Aaron check this link under repetative member, this may help.

   http://www.southernpine.com/designvalues1.htm

1. 
   Jeff_Clarke_ | Dec 13, 2000 04:46pm | #2

   *Hold on guys, that's not what repetitive member means! It specifically means load values for a wood member used within a system of wood members, rather than individually.Aaron - Here's the 'duh' of the day - you won't find span tables for doubled and tripled members specifically because they are assumed to be headers and the loads therefore depend on the tributary area being loaded. For instance, your deck edge band could be spanning 12 feet but could be loaded any number of different ways by the deck framing. If deck joists bear on it are they 10 feet long or 50 feet long? If you find tabular values for a single member in load capacity (lbs/lin. ft. capacity) you may double them for a double member, so a joist that can carry 60 lbs/lin. ft. when doubled can carry 120 lbs/lin. ft. but the shear values don't change, nor does deflection.If this doesn't make sense to you, you might want to get some local professional assistance.Jeff

1. 
   Dan_Metzcus | Dec 13, 2000 06:00pm | #3

   *Jeff does it matter if the members are 16" away, 19.2" away or no inches away to become i a system of wood members ?

1. 
   Jeff_Clarke_ | Dec 13, 2000 06:28pm | #4

   *That's not the point. It matters if they are used as part of a uniform floor or roofi systemor as a i headerwhich collects loads! In a floor or roof system, the very fact that there is an even spacing in one direction allows for span tables in the first place.Headers (beams) can be loaded in all sorts of unpredictable ways and therefore you won't find them in span tables unless (as in LVL and PSL tables) there are lots of assumptions noted, like snow load, roof + 1 floor, DL + LL , continous over supports, etc. etc.You don't use floor/roof span tables to size headers, period.Jeff

1. 
   Dan_Metzcus | Dec 13, 2000 07:03pm | #5

   *We are talking about a pretty i predictabledeck are we not?

2. 
   Jeff_Clarke_ | Dec 13, 2000 07:11pm | #6

   *Dan - No offense intended but you miss the point. The perimeter beam could have no joists bearing on it, in which case it is treated like any other member, except maybe having a bit more dead load from railing construction, etc, OR, it could have joists hanging on it in which case it is a perimeter beam that must be sized, not looked up in a table.Bunch of nonsense? Well keep in mind that the design load for decks here (NJ) is 100 pounds / square foot and that there have been several catastrophic deck collapses in recent years notably one at the Jersey shore last year in which there were a number of severe injuries. The problems are more frequently due to poor engineering and poor assumptions rather than poor construction.In case you think I'm full of it - read this or read this or this. Decks should not be engineered by the seat of the pants.Jeff

3. 
   Dan_Metzcus | Dec 13, 2000 07:20pm | #7

   *I apologize for missing your point Jeff. Still waiting for one of my decks to collapse.

4. 
   mags | Dec 13, 2000 09:12pm | #8

   *Jeff, I agree with what you say, but could you clarify somehing for me? You said:"a joist that can carry 60 lbs/lin. ft. when doubled can carry 120 lbs/lin. ft. but the shear values don't change, nor does deflection."Wouldn't the shear and moment (deflection) capacities also increase in a doulbled up joist because of the increased cross-section? (I vaguely remember bits and pieces from my structural design classes quite a few years ago.)Mike

5. 
   Jeff_Clarke_ | Dec 13, 2000 10:09pm | #9

   *mags00 - Good questions - I mispoke regarding deflection and shear.Deflection for a simple beam or joist is governed by the formula 5wl^4/384EI - where w is the uniform load in pounds per unit of length, l is length of the beam in inches (l^4 means 'l to the fourth power') E is modulus of elasticity based on species and I is moment of inertia, which is based on the cross-section. (I=bd^3/12 where b is width of beam and d is the depth). Note from the moment of inertia formula that a change in depth will have a much greater effect than a change in width. Nevertheless, for a given load, doubling the width does cut the deflection in half if the two beams act together.For shear, my point was that the i maximumhorizontal shear stress (say 90 for Dfir) for a specific species and grade is not helped by having multiple members - you can't expect the maximum capacity to go to 180 - it has to remain under 90. You are correct however that the capacity to resist shear is doubled (fv=3V/2bd where b is doubled, fv is halved).Jeff

6. 
   Mr._Pita | Dec 13, 2000 10:13pm | #10

   *>The problems are more frequently due to poor engineering and poor assumptions rather than poor construction.I'd actually argue the problems are more frequently due to idiotic party hosts, packing 30 or 40 people on a deck that's only 10x20. I know the collapses you speak of, seems 1 per month during the summer, or at least 1 per season. I'm amazed the hosts aren't criminally charged or sued.I know what you meant, sorry to rant, but what ARE they thinking?

7. 
   ADM_ | Dec 14, 2000 03:54am | #11

   *Thanks for all the info, Jeff. The deck is double 2x10 band, 2x10 joists set flush inside the band. deck is roughly 12'x30', held up with 6x6 posts notched and bolted to the band. I remember reading a chart from somewhere that did show spans for doubled and tripled 2x10,and 2x8s. That chart showed that with a doubled 2x10 band I could span 7'6". I just can't seem to remember where the chart was.

8. 
   Jeff_Clarke_ | Dec 14, 2000 03:07pm | #12

   *True, but if the deck is designed to suit code-required loads and is properly attached, it shouldn't be a problem. A 10 x 20 deck that can support 100 PSF can carry 20,000 pounds - 40 people therefore shouldn't be a problem unless they all start jumping up and down (dynamic loading) at the same time. Even at 40 PSF (same as residential interior in most places) that deck should be able to safely carry 8,000 pounds, or 40 200-pound people.The main problem is, decks are treated as if they were outdoor living rooms but frequently are not built up to the same structural standards as (enclosed) living spaces. Or, alternately, people assume that if a deck is suited for use 4 feet above grade it can be raised to 20' above grade without further consideration (cross-bracing, kl/r column ratio, etc.) - that leads to disaster.Jeff

2. 
   ADM_ | Dec 14, 2000 03:09pm | #13

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   I'm trying to find a span chart that shows a table for doubled and tripled 2x's, more specificaly for a doubled up treated deck band. The charts at Southern Pine Council don't seem to have them. Any other tables out there?