The Dean of Home Renovation & Repair Advice

# COMMUNITY FORUM

latexia707

03:10PM | 04/11/07
Member Since: 11/01/06
Hello to all!

I have a problem that has been grinding my brain cells for many years...

How do you calculate the angle and length of cross-bridgings?

Say I have 2 joists 25" apart, and are 3"x11". I want to 2"x4" as cross-bridging.

I can do it if I simulate the 2 joists adjusting the distance between them and with a pen mark the line what is given by the simulated joists...

But good Lord... how did the carpenters a 100 years ago calculated it?

I know I couls cimply take the 2"x4" and place it horizontaly between the joists... but that is not the way I want to go... I want to go 'old school/fashion way'.

Many thanks in advance!

doug seibert

05:31PM | 04/12/07
Member Since: 08/10/02
The basics were devised over 2,000 years ago with Euclid (300 BC)

Here's a book from 1910.......Read the chapter "The Steel Square"

http://journeytoforever.org/farm_library/device/devicesToC.html

Using your example you could construct a triangle 11" high and 25" long and compute the length of the diagonal and angles using triginometry and geometry......

The angles on each end of the cross brace are equal and ends are parallel

An easy NoMath method is to draw the cross section and just draw the required bracing "by-construction"

Retro-fitting "cross-bracing" in an existing structure is kinda tough......the top nails are normally driven while the floor structure is open (the bottoms are nailed when the floor is loaded)......trying to hand nail the tops of the diagonal braces in the joist bays is a challenge

"...measure once.....cut twice....throw that one away and cut a new one...."

latexia707

04:58PM | 04/13/07
Member Since: 11/01/06
my HERO!

many thanks!

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