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Q: sizing I beam ( No Answer,   2 Comments )
Subject: sizing I beam
Category: Science > Physics
Asked by: texasfarmer-ga
List Price: $8.00
Posted: 18 Apr 2006 07:59 PDT
Expires: 18 May 2006 07:59 PDT
Question ID: 720171
I would like to create a substructure for the roof of my house that
will allow me to remove interior and exterior walls.   The
roof is 24x 44 feet and made of wood and tar shingles.  My plan is to
use two 44 foot ibeams along the length of the 44 foot sides.  Can
someone help me calculate what size i beams i need?  One individual
told me that for the purposes of calculation, the beams will have to
support 480 pounds/foot across the 44 foot span.  Any links to tables
describing load capacities of I beams would be appreciated.

There is no answer at this time.

Subject: Re: sizing I beam
From: redhoss-ga on 18 Apr 2006 09:32 PDT
If you remove interior load bearing walls, then the roof trusses are
unsupported for the full 24'. If they are not designed to span 24',
you have a problem.
Subject: Re: sizing I beam
From: toufaroo-ga on 02 May 2006 14:13 PDT
First off, if I were you, I would get a licensed structural/civil
engineer to help you out.  You can find one on your state's
Professional Engineers website or in the phonebook.

Having said that, here's how you'd go about calculating the required

First, you need to determine the moment in your beams.  Assuming 480
lb/ft is the correct load, then your beam moment can be calculated at
M = wl^2/8, where w is your load in lb/ft and l is your beam length in

So, that gives you 480 * 44^2/8 or 116160 ft-lb.  This is your bending
moment, abbreviated as "M".  Note that this number SHOULD already have
a safety factor built into it (assuming the individual who gave you
the 480 value knew what he was doing).

Next, you need to size your beam to accomodate this bending moment. 
The key piece of info here is the Section Modulus, also abbreviated as
"S".  This is given in tables.  If you know the dimensions of your
I-Beam, you can also calculate this.

Finally, you need to know what type of material you are using.  A
steel I-Beam will support much more weight than an engineered wood
I-beam of the same dimensions.  The value you are looking for here
(found in the manufacturer's list of properties) is the ALLOWABLE

Your final equation becomes 


As long as this criteria is met, you're OK for stress.

Next, you need to be OK for deflection.

For a roof, deflection needs to be less than L/240, meaning
44ft*12inches/foot/240 = 2.2 inches at midspan

Actual deflection is 5wl^4/384EI

E is Young's Modulus, and is dependent on material properties.  It is
roughly 30 x 10^6 psi for steel.

I is your moment of inertia.  This can be found in tables of beam properties.

w and l are the same as before.

As long as ACTUAL DEFLECTION < 2.2 inches, you're OK for deflection.

If you're OK for deflection and stress, then your beam size is
appropriate.  Pick the smallest beam that meets both criteria.

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