Cob Re: 2 x 4 I beams

[Robert W. Tom]

on Tue Dec 22 1998
goshawk at gnat.net ("Patrick Newberry", the Mauk Mudder) wrote:

[snipped & pasted]

>    I probably should be singing Christmas carol's or  something 

>    My roof is a masonary roof...

>    Are  the three 2x4's per "beam" an over kill. (spans will
>    from 12 to 16 feet) But they can be built from used lumber and
>    pieced together so I can span the 16 feet with 8 ft length
>    lumber. 


Hello Pat and TALL Murderers;

There's really not enough info (ie specified loads (live and dead),
lumber grades, roof configuration etc) above to give you an answer
on the performance of the "I" beams you are proposing ... but offhand,
without doing any number-crunching, I would say that they would
probably be under-sized.

Generally speaking, the bending resistance of any flexural member
is largely dependent on the distance of the top and the bottom
edges (compression & tension flanges respectively) from the middle
(or neutral axis)... ie  the greater the distance from the neutral
axis,  the stiffer the member.

For floor joists spanning 12 ft, 2x8s are just barely adequate. 
Again without crunching any numbers, I think that the 2x4 I-beams
(@ 6.5" deep ?) wouldn't be much stronger than a 2x8, and probably
not as strong as a 2x10.

With thin masonry roof cladding, I would think that deflection
would want to be kept to a minimum to avoid cracking.

On an I beam, much of the material in the web can be eliminated (ie
look at parallel chord trusses ,open-webbed steel joists or large
steel I-beams with round cut-outs in the web)... so in a 3-2x4 I
beam, one of the 2x4's is under-utilised.

You would be better-off making the beams 12" or 16" deep using
plywood or OSB gussets to space the top & bottom chords. This has
the added advantage of providing you with deeper cavity for
insulation (in addition to the superior strength).

Also, steel gusset plates (20 gauge or so ?) should be applied to
both sides of the butt-joint of the 2x4s used to make the 12'-16'
chord members. The butt joints should NOT be placed at mid-span,
but rather, at the 1/3 or 1/4 span points (ie points of zero
moment). The grade of lumber used for the bottom chord (ie tension
flange) is critical, and using a continuous member of #2 grade or
better lumber would be prudent.

One of the beauties of box beams and plywood web beams is that they
can be built in situ, making irregular profiles a breeze. ie The
top chord & bottom chords can be propped-up to form wonky curves
that would delight any funky-gooey-architecture fan and then one
simply applies the joining web afterwards... and voila !... a truss
(or box beam).

All that being said, I would strongly urge that you have an
enginoidal sort look at your proposed beam design once you do get it
drawn up.  Either that or do some Power Studying at your local
university library to bone-up on beam and truss design.
Collapsing masonry roofs are not a Good Thing.

Okay, now back to singing those carols. A one an' a two an' a three...

Thengya. Thengyallverramuch.
--
Rob  Tom
---------- * ------------
be417 at FreeNet.Carleton.ca
Kanata,  Ontario,  Canada