Understanding structural sheathing & bracing

[jraabe]

This is another article by Paul Fisette (see beam article below).

http://www.umass.edu/bmatwt/publications/articles/bracing_foam_sheathed_walls.html

It explains the forces at work on the exterior of the house and why structural sheathing (OSB or plywood) gives so much strength to a building.

When exterior foam board replaces this sheathing more sophisticated bracing is needed to hold the house together when the wind blows.

PS - Cick the "List by Titles" link on the upper left for more well done articles on topics such as OSB vs plywood, cellulose insulaton vs fiberglass, details that keep walls watertight, and others.

[Amanda_931]

So the standard bracing used in the 70's with black-board was fine.  Pieces of plywood the same thickness as the blackboard in the corners.  

[jraabe]

Plywood at the corners gives basic structural rigidity. Alone this is not usually enough for earthquake areas and it is not as good as plywood or OSB sheathing over all the studs.

[Amanda_931]

Somewhere around 100 miles W of here one starts getting into a lot more earthquake possibility--the New Madrid fault.

But while there was a minor earthquake in the county some months back, Middle Tennessee doesn't worry much about them.

We could be wrong.

We do get tornadoes.

[jraabe]

For 99.99% of its life a house only has to deal with downward loads. This is what we call simple static loads.

It's the sideways and upward dynamic loads that happen so very infrequently that can spell doom for a house. The extent to which you need to design and build for this varies considerably not only by the big picture earthquake, wind and water forces of your region, but by the very local sheltering or non-sheltering aspects of your site.

There are a few simple concepts that will help tie the structure together inexpensively:

• Try to get a two stud bay (2'-8" or 4' wide) panel of structural sheathing for the full height of the wall at both sides of all major corners. Nail this well to studs, plates and sills. Have horizontal panel joints meet over solid framing and nail both sides of the joint. You are building a box beam and adhesives will add even more rigidity.
• Use metal hardware such as straps, framing anchors and bolts to tie the wood framing to the foundation, the floor framing to the walls and the walls to the roof framing. You want these parts of the structure to work together and reinforce each other - especially at the corners.
• Check that all roofing, siding, sheathing and framing is installed with the right type of fasteners and at the right spacing. Many of the Katrina houses failed because framers and roofers used short staples or too few nails to keep things from blowing away. Nails are cheap insurance - use lots.