Messages and Online Forum Q&A

Re: Steel Beam in Balloon-Framed Home

From: matnyc2908@aol.com
Category: Structural Design
Remote Name: 70.23.230.65
Date: 27 Feb 2006
Time: 09:43 PM

Comments

Question: I have an old balloon-frame home with sagging second-floor bedroom floor joists. Floor joists run 20' (true 2x8) on 12" centers. They run through two bedrooms side to side 16' deep with a wall in the center that helps support the ceiling joists that run 20' long as well (they do not overlap on this wall). The living room is under all this with no center support 20'x16'. I do not want to put a beam under this; rather, I want to install it in the floor. I will remove the center wall and remove all floor joists and install a Wide-flange beam and tie the floor joists into the web of the beam. The beam will cover a 16' span and carry the weight of two bedrooms 40lbs sq. ft. load. and an attic of same size that will be used for storage so I figure 30lbs. sq. ft. load. The roof does not transfer any weight to these joist. I had wanted to install a W840 or a W848, but could not find the load capacity of any of these beams. Any help would be appreciated. Thank you for your time.

Builders Websource® Answer: [Complete answer pending further clarification from customer]. Assuming a 40 psf live load and a conservative 15 psf dead floor load, a quick analysis of your existing floor joists (2x8 @ 20' on 12" centers) confirms that your existing floor violates virtually every measure of structural integrity including bending and deflection -- and this is before factoring in the load from the center partition, which also carries your second-floor ceiling and attic loads. This must feel like a trampoline when you walk up there!

You have the right idea to install a center beam perpendicular to your existing floor joists, which will effectively cut your 20' span down to 10' for the existing floor joists. Keeping everything else constant, the existing 2x8 floor joists will feel plenty stiff assuming the effective span is cut in half.

As to sizing the steel beam, you need to know the total load on the beam. Assuming no roof load, the beam must carry the following three loads:

• The tributary floor live/dead load
• The wall partition dead load
• The tributary ceiling/attic load

In your case, the tributary area for both the floor and ceiling is 10' * 16', or 160 square feet.  Using 40 psf live load and 15 psf dead load, the uniform load on the beam from the floor is 400 plf (live load) and 550 (live load + dead load). Assuming the wall partition is 8' high and framed from typical 2x4 construction and 1/2" drywall on both sides, the wall weighs approximately 6 psf, or 48 plf for an 8' high wall. The full weight of this wall bears down directly on top of the proposed steel beam. Finally, let's assume that the attic has a 30 psf live load and 10 psf dead load, this adds another 300 plf live load and 400 plf live load + dead load to the beam. Putting this all together yields a total live load of approximately 700 plf and a live load + dead load of approximately 1000 plf. Running this through BeamChek yields a range of W8 options...however, using these particular figures, you'd need a W8x48 to keep deflection to an acceptable level. A slight camber is also recommended.

Please note that this size beam is approximately 8.5" deep, so you'll need to make sure it's not sticking through the bottom of the ceiling. Finally, this analysis does not include any seismic or wind load considerations, nor does it address required foundation analysis, anchoring details, integration/fastening to existing floor joists, number and size of bolts to tie into your floor joists, etc. This information is for educational purposes only. To avoid a repeat of your current problem, you must consult a licensed structural engineer who can perform an actual site analysis of your situation and make specific recommendations to transfer loads properly.

Article Tools

Builders Websource® is a corporate member of ASTM International—a non-profit engineering body dedicated in part to the standardization of building materials specifications, products, systems, and test methods for improved health, safety, and reliability of residential and commercial structures.

Copyright © 1999 - 2008 Builders Websource®, an InBuild™ company. All rights reserved.