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Old 31st August 2008, 03:55 PM   #1
Tony Bryer
Join Date: Apr 2008
Location: Melbourne, Australia
Posts: 832
Default Effective lengths of timber posts and columns

A query first posted on our now-removed helpdesk

Hi, i am struggling with the wording in the manual regarding the effective length/actual length ratios regarding timber posts. Please could someone explain the difference between restrained in direction and restrained in position. Examples would be very helpful. In my particular scenario, i have a ridge beam supported by a 100x100 post down on to a load bearing wall, the other end is built into the wall.

Thanks In advance

Hopefully this will help, but as ever anyone else who wishes to chip in is welcome to do so.

'Restrained in position' means that at that bearing the stud or post cannot move laterally. This will be the case in most building structures.

'Restrained in direction' means that at the bearing or post end there is some form of fixing that will stop it rotating at that point.

Most studs, posts etc are restrained in position but not in direction, i.e. at the ends there is some nominal fixing to stop them moving but it provides no angular rigidity. As you load the member it tends to bend and the ends rotate but cannot move sideways. The effective length is therefore 1.0L

Consider a table top on four legs - no magic to four, just easier to visualise:

Case 1: the legs are firmly fixed to the top so that at that point they cannot rotate and are cemented into holes in the ground. Some other form of restraint stops the top moving sideways. Both ends are restrained in direction and position - Le = 0.7L

Case 2: as case 1, except at the bottom the legs rest on the ground and are fixed to it by nominal brackets that stop them moving sideways but not rotating. Both ends fixed in position (because the top cannot move), one end fixed in direction. Le = 0.85L.

Case 3: (as for the studs): the legs are nominally fixed to the ground and to the top. Le = 1.0L

Case 4: Like case 1 but the the top is now free to move sideways. Restrained at one end (the bottom) in position and direction and at the other end in direction, but not position. Le = 1.5L

Case 5: Restrained in position and direction at one end and free at the other end. Classic example here is a flagpole.

Hope this helps rather than confuses!
Tony Bryer, Greentram Software Pty Ltd
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