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WCB Standards

WCB Standard: G602 Log Loader and Log Yarder Raised Cabs, March 1990

© Workers' Compensation Board of British Columbia. All rights reserved. No part of this document may be copied, reproduced, or distributed for profit or other commercial enterprise, nor may any part be incorporated into any other publication, without written permission of the Workers' Compensation Board of B.C.

1. General
1.1. Scope

This standard pertains to log loaders and log yarders, and other mobile equipment where the operator may be exposed to hazard caused by intruding or flying objects, such as whipping cables, loose debris, snags, tree trunks, limbs, etc. It is supplementary to G601 Standard for Log Loader and Log Yarder Backstops, and is supplemented by G603 Standard for Log Loader and Log Yarder Window Guards.

1.2. Purpose
This standard is intended to describe the minimum requirements for the design and selection of plates, framework and supports for raised cabs on equipment without a backstop. This will form and provide a protective structure for the operator inside the cab.

2. Design Principles and Assumptions

The following points are stated to clarify the underlying principles and assumptions of this Standard.

A static force resistance design criteria is stipulated to ensure that intruding or flying objects will not deflect the cab beyond a certain limit.

Also included is an energy design criterion, since in an actual situation involving flying objects, loading will be dynamic and possibly impact. Hence the adequacy of the structure is related more to energy absorption capability and details of weld design and welding procedure rather than static strength.

The recommended design horizontal and vertical force will not necessarily duplicate the force imparted by an actual flying object such as trees, whipping cables, etc.

As the cab elevation is increased, the vertical load requirement 3.1.1 may be reduced accordingly.

It is assumed that if the cab/structure can resist a force of W (Appendix A), then it will have adequate resistance to whipping cables. A magnification factor has been incorporated into the formulae to compensate for very small logs, because in such cases, other factors such as flying debris or cables may govern. In any case, W used for design purposes should not be less than 9000 N (2000 lb).

Although cabs meeting these criteria may not deflect within the Deflection Limiting Volume (DLV See SAE J397a) under all circumstances, it is a minimum requirement for the Cab Protective Structure to have a "Crush Protection" design capability to withstand the force exerted on it by a hypothetical blunt log striking end-on at a velocity of 11 km/hr.

Furthermore, there is an impact or strength requirement which is intended to ensure that all members of the cab will have adequate resistance to brittle fracture under cold temperatures.

Finally, there is a visibility requirement which is to ensure that the operator's vision will not be seriously obstructed.

The operator protective structure can be an integral part of a cab or can be a "cage" outside an existing cab. Hereinafter the term "Cab Protective Structure" shall mean any guard that envelopes the cab or any guard that forms part of a cab. (See Appendix D.)

3. Design Loads

3.1. Vertical Load Requirement

Force Resistance Fv = 2.5 x Wxi units N, m
  Fv = 8.25 x Wxi units lb, ft

H = distance from grade to top of Cab Protective Structure.

W = weight of log handled as defined in Appendix A.

Fv = vertical equivalent static force.

i = impact factor as defined in Appendix B or other acceptable method.

Energy Absorption

Ev = 0.152 W

units N, J

Ev = 6W

units lb, lb-in

Ev = ultimate energy to be absorbed by the frame at the point of impact. (J), (lb-in).

3.2. Lateral Load Requirement

Lateral Load Requirements 

FL = equivalent lateral static force, where FL is greater than the tipping force, then FL may be reduced to that of the tipping force.

4. Design Requirements

4.1. Frame

All frame members of the Cab Protective Structure shall be designed to resist the applied load in accordance with Appendix C or with some other acceptable design criteria to the Board.

In addition, the frame shall be designed to absorb the impact energy as given by Ev and EL of 3.1.1 to 3.1.2.

4.2. Cab Skin
All exposed unguarded sides of the cab should be protected with metal plates

or other suitable material. They shall be designed to resist the FL as defined in 3.1.2.

4.3. Supports
The cab protective structure shall be secured to the structural parts of the (carrier) main frame of the log loader or log yarder. Such structural parts shall be adequately reinforced if necessary to resist the loads imposed on them by the cab protective structure.

4.4. Fastenings
If bolts and nuts are used in the fabrication of the guard, they shall conform to or exceed the ASTM Designation A325 Requirements.

4.5. Visibility Requirements
The cab shall be equipped with adequate view areas. All such view areas shall be guarded in accordance with WCB G603 Standard for Log Loader and Log Yarder Window Guards.

4.6. Impact Strength Requirement
All members of the guard shall be made of material with good impact absorbing properties. The following guideline may be used:

  • Low carbon content (less than 0.28%).
  • High ratio of manganese to carbon.
  • Low phosphorous content.
  • Fine grain size.
  • Heat treated.
  • High ultimate energy resistance.

Examples of steel meeting or exceeding the above requirements:

  • ASTM A36, CSA G40.21 33G, 44W - for plates, bars and angles.
  • CSA G40.21 50W - for HSS. (Hollow Structural Sections)

4.7. Weldment
Weldments shall conform to applicable sections of General Specification for Welding of Steel Structures, CSA W59.1-1970 (or latest revision thereof) and shall be performed by licensed welders only.

4.8. Alternate Exit
The operator protective structure shall be provided with an alternate exit. Such exit shall have a minimum clear opening of 60 cm diameter (24").

4.9. Glazing
Only safety glass or other suitable material with similar shatter-resistant characteristics shall be used for window areas.

4.10. Sound Isolation and Absorption
The cab interior shall have a sound level reading of not more than 80 db under normal working conditions and with all openings closed. Flammable sound absorption material should be avoided.

Derivation of W.

The symbol "W" represents weight of the heaviest log expected to be handled by the log yarder or loader. For the purpose or this standard, g (density of logs) should be assigned a value of 560 kg/m3 (35 pcf) which is the arithmetic mean of the common species found in B.C. The following formula may be used to compute the expected weight of incoming logs:

Formula to compute the expected weight of incoming logs.

DB = butt end diameter m, ft.

DT = top end diameter m, ft.

L = average length of logs handled, m, ft.

g = density of logs handled kg/m3, pcf.

Impact Factor* (i)

Formula for Impact Factor* (i)

v = velocity of approaching log.

ΔST = static deflection of guard member due to weight of approaching log.

g = acceleration due to gravity (32 ft/sec2 or 9.8m/sec2).

* Other acceptable method may be used.


Diagram of Loading Conditions for Cab Frame Design (WCB-G602)

Diagram of Reinforced Cab and Window Guard (Typical)

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