[TheForge] Wooden Beam strength for Rigging

Mon Jan 30 19:47:21 EST 2006

Ouch Bill.... 25' is pretty far.  Obviously an older building.  That's why
the unusual size.  Your 3x12 @ 24" roughly approximates 2x12 @ 12".  My
floor joists charts don't go to 25' but 2x10 @ 12" in a really good wood
species spans 24'.  Therefore and without doing manual calculations, I think
your right on the limits of your structure.

When you work with larger spans like this, there is always a chance of
having other problems.  I think I would look at beefing up a couple of your
joists.  As I said, you could do that with some fletch plates.  It would be
a little hard to add plywood fletches and get enough nails in so I'd think
about steel.  That could be done with plate, channels or even angles.  The
problem with plate is that it has no stiffness and you don't need additional
strength in the middle of your rafters, you need it top and bottom ....
mostly bottom.  I'd think about using a couple of angles or channels on
either side of the joist to be strengthened and bolt them so they are at the
bottom of the joist.

 |
]|[ 

Another possibility is to look at adding a bottom tension member in the way
of a rod or cable.  I have an oak 12x12 x 25' beam in my barn that has a
5/8"dia rod added to the bottom like a truss.  The rod is located about 4"
below the beam and turns up at 30 degree angles to go through the beam at
the ends where it has washers and a nut.  The rod is held off the bottom of
the beam with oak posts at the rod bending points.  A crude but effective
truss.  

The problem with the steel is your cost.  If you have headroom, can you add
a deeper member beside the rafter.  I would consider some built up plywood
box beams with shallow bearings.  (Not as deep at bearing so they will sit
on your existing sill.  You can build a strong beam using 2x4, 1/2" plywood,
glue and nails.

Walt

-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Woolley
Sent: Monday, January 30, 2006 4:25 PM
To: Sponsored by ABANA
Subject: Re: [TheForge] Wooden Beam strength for Rigging

Walt,

I'm saying beams or roof joists. They support a flat roof that sits right on
top of them.  The span is 25' with a 6" slope over the 25' span.  Most are
in pretty good shape.  A few have some splitting and twisting but not much
that really concerns me.  I'm thinking 6" beam just for ease of installation
and I really don't need to be able to lift alot of weight.  Like I said, 1/2
ton max. but I didn't look into whether or not a 6" beam would give that to
me. I'm just throwing that out there as a starting point.  I'd like to place
them at 1/3 and 2/3 of total span.  Maybe bridging the two now and again to
support something placed on or over the table which will be in the middle of
the span.  I could easily leave at least 6" of wood under any holes I would
make for the hangers.  What are fletch plates?  Thanks.  BTW I poured a
footing for a gantry crane elsewhere in the shop.

Regards,
Bill Woolley

----- Original Message -----
From: "Walter Mullett" <wmullett at bright.net>
To: "'Sponsored by ABANA'" <theforge at mailman.qth.net>
Sent: Monday, January 30, 2006 10:05 AM
Subject: RE: [TheForge] Wooden Beam strength for Rigging

> Bill,
>
> Not enough info here.  You say the beams are 3x11.5 but you don't give 
> span
> or slope.  You also say beams and then you say rafters.  What are they?
> - Is this a sloping roof with ridge or single slope? (If you have a ridge,
> do you have collar ties on your rafters? - Over-loading a ridge rafter
> system can push out the sidewalls and sag the ridge.)
> - How far from the end of the beam/rafter do you want to hang your beam? (
> Mid-span is a worse condition than 1/4 span. )
>
> Given the above .... You really aren't talking about a lot of load.  If 
> you
> are living in a snow environment, your roof will probably support at least

> a
> snow load of 30lb/sf.  If you can spread your load over +/- 30 sf, you 
> could
> just be replacing that load.  However, I doubt your 6" 'I' will do that.
> You can test out that by loading the beam on the ground and seeing how 
> much
> deflection it has.
>
> Your existing 3x members are probably stout enough to take up the point
> loading they will have.  My concern with these would be to make sure your
> connections are in the middle of the side of the beam.
>
> You can build it then load test it.  Wood is great because you have a
> running factor.  Unless you have some really bad flaws (knots - holes in
> wrong locations), it deflects and groans a lot before it fails.  Just 
> check
> your deflection as you add load.  If you get too much deflection, you can
> sister the joists or add fletch plates to them.
>
> Walt
>
> -----Original Message-----
> From: theforge-bounces at mailman.qth.net
> [mailto:theforge-bounces at mailman.qth.net] On Behalf Of Woolley
> Sent: Sunday, January 29, 2006 10:49 PM
> To: theforge at mailman.qth.net
> Subject: [TheForge] Wooden Beam strength for Rigging
>
> Greetings,
>
> I seem to remember a thread about beam strength a while back but don't 
> have
> any info.  I'm not sure if this is the right forum but here goes.  I want 
> to
> hang small steel I beams(6"?) under my roof joists to rig from. I'm hoping
> to get up to 1/2 ton or so capacity.  The beams measure 3x11 1/2 with no
> room above (obviously butt up to roof deck), so I have to drill to bolt
> through or lag in to hang straps down and under to support steel.  The
> joists are spaced 2 feet apart. I hope to span at least 5 or 6 of them. 
> Does
> anyone have a known reference for engineering specs on this or any 
> practical
> experience doing something similiar?  Any help would be appreciated.
>
> Regards,
> Bill Woolley
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