Timber Joists
Timber Joists
(OP)
Hey,
Im undertaking some research at the moment which aims to determine how load is spread between timber floor joists. In order to achieve this I have made a floor joist system which consists of five joists. I have loaded the central joist in the system with two point loads at the 1/3 points along it's lenght. When testing the joist system I recorded deflections on each joist and also strain. It turns out that the strain gauges will not bond to the joists properly and this led to mis- reading of strain on each joist. All I have left is deflection readings to work with and im not sure how to accuratly determine the degree of load spread using just deflection. Also I cannot use Young's Modulus in any way as the system was only partially composite (joist- timber flooring)........any ideas on how to determine load spread using deflection? Thanks!
Im undertaking some research at the moment which aims to determine how load is spread between timber floor joists. In order to achieve this I have made a floor joist system which consists of five joists. I have loaded the central joist in the system with two point loads at the 1/3 points along it's lenght. When testing the joist system I recorded deflections on each joist and also strain. It turns out that the strain gauges will not bond to the joists properly and this led to mis- reading of strain on each joist. All I have left is deflection readings to work with and im not sure how to accuratly determine the degree of load spread using just deflection. Also I cannot use Young's Modulus in any way as the system was only partially composite (joist- timber flooring)........any ideas on how to determine load spread using deflection? Thanks!






RE: Timber Joists
RE: Timber Joists
BA
RE: Timber Joists
For the percentage of the total load taken by any particular joist, divide the deflection of that particular joist at the centerline of its span by the sum of the midspan deflections of all five joists. It's just that simple. No need for all the fancy stuff...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Timber Joists
E varies significantly between two pieces of wood (heck, it even varies along the length of a single piece of wood). We use a relatively conservative value of E for design but, at best, it could be considered an "effective" modulus. That's perfectly OK for design, but not for research if one is attempting to understand something a bit more fundamental.
RE: Timber Joists
BA
RE: Timber Joists
You can then do a statistical analysis to compare the average comparitive deflections. Remember that you have two sample for each load as the distribution should theoretically be a mirror image on both sides.
Also it is not usual practice to treat the joists as a composite T beam even if they are glued to the floor ply. This is usually done to minimise vibration rather than for strength.
RE: Timber Joists
RE: Timber Joists
BA
RE: Timber Joists
RE: Timber Joists
RE: Timber Joists
As it is the deflection results give as good an indication of relative applied load as the strain gauges would, if not better.
The deflection measurement will average out any stiffness variability along the joist length, which the strain gauge won't.