Model Engineers’ Workshop – July 2019

The M8 x 70mm hexagon headed coach screws

to use the lift ing beam when fallen snow is present on the roof.
Similarly, pools of water on the roof due to poor drainage will also
increase the static load on the roof beams. Also, if you ever sell the
house (inevitably, either you or your executors will sell the house)
the purchaser will know exactly what the beam safe working load is.
The new purchaser may not need to use the beam, but at least you
will know that no harm will come to anyone through ignorance of
the load capacity. In my case, I added a notice to the dividing wall to
the eff ect that it was an integral part of the lift ing beam support, so
should not be removed unless the lift ing beam were removed fi rst.

Conclusions
A method of designing a safe attachment method for a lift ing beam
to substantially horizontal wooden roof beams is described.
A practical experimental method is given to establish the Young’s
Modulus of timber beams.
A design method is given for calculating the allowable load on a
timber beam, the defl ection and the maximum stress.
A design method is given for determining the allowable load for
a vertical timber column to prevent failure by buckling or lateral
instability.
The safe loads for coach screws into wooden beams are given
along with the minimum spacing required. ■

Nomenclature:
A column area, sq.mm
C height of column, mm
D Shank diameter of screw, mm
d depth of beam; least dimension of cross-section; mm
E Young’s modulus N/sq.mm
F Static load, N
G Specifi c gravity
H Length of penetration of the threaded part of the screw shank,
mm
I Moment of inertia, mm4
L Length of beam, mm
M bending moment, Nmm
p Pull-out load, N

Q Allowable load, N

r least radius of gyration, equals d/√ 12

w width of beam, mm

y Defl ection, mm

?s allowable compressive stress; maximum stress in bending

References

Wood handbook—Wood as an engineering material, U.S.

Department of Agriculture, Forest Service, Forest Products

Laboratory General Technical Report FPL-GTR-190. Madison, WI,

(2000). Available for download from: http://www.fpl.fs.fed.us.

Roark, R.J., & Young, W.C., Formulas for Stress and Strain, 5th Ed.,

McGraw Hill Book Company, New York, (1975).

Cain, Tubal, Model Engineer’s Handbook, 3rd Edition, Nexus Special

Interests, (1996).

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