OVERVIEW
ABSTRACT
Work to revise
the Canadian National Safety Code standard for load securement
identified a need for research into the mechanics of load securement
systems, and led to an international research project supported
with public and private funding from both Canada and the United
States. Here is an outline of the issues arising from the four
elements of load securement systems, anchor points, tiedowns,
blocking and friction — and their interactions with loads
of specific characteristics. The results will be compiled as principles
written in clear language to serve as a basis for development
of a single North America-wide load securement standard, effective
July 1st, 2005, in Canada.
INTRODUCTION
Security of
loads on vehicles is a matter of public safety, and is therefore
subject to government regulation and a body of industry practice.
Regulations on loads on vehicles are set and enforced by the provinces
and territories in Canada and the states in the U.S., over sixty
jurisdictions, many with more than one agency involved. Even though
the U.S. federal responsibility for inter-state commerce has resulted
in a large measure of regulatory uniformity, there are still significant
differences in requirements, interpretation and enforcement that
create problems for truckers. These are at least hindrances, if
not barriers, to the free movement of goods, and thus increased
transportation cost.
Canada's National
Safety Code required an updated standard on load securement, and
the Canadian Council of Motor Transport Administrators (CCMTA)
was charged to develop it. Their task force reviewed current regulations,
but could not resolve some significantly different requirements
between provinces and territories because there was no ready means
to evaluate the capacity of load securement systems. The task force
therefore produced a framework for the standard, and identified
a number of areas for research before it could be completed [1].
Ontario Ministry of Transportation (MTO) prepared a draft research
proposal from extensive consultation with staff and industry,
and circulated it widely for review throughout North America.
A technical committee, convened from government agencies and industry
associations representing trailer and equipment manufacturers
shippers and carriers, considered the review comments and finalized
the research proposal [2]. It was approved as a project by CCMTA
at the end of 1993, funding was secured, and a management committee
was formed to steer the research. The goal was broadened from
a Canadian Standard to a single uniform North America-wide standard
when the project received technical support and funding from governments
and industry in the United States.
Here are the
issues identified by the CCMTA Task Force, and a description of
the program to address them.
IDENTIFICATION
OF ISSUES
This section
summarizes the issues and areas for research identified by the
CCMTA Task Force [1] discusses them, and notes how it was proposed
to deal with them.
General
Requirements
The task force
was concerned about the general requirements for all loads. These
included the performance expectation for the standard, the adequacy
of a design deceleration of 0.6 g, and whether the current requirement
that the aggregate working load limit (WLL) of all tiedowns exceed
the weight of the load is appropriate.
These issues
were considered beyond the scope of the research proposal. They
were, nevertheless, important aspects of the discussion of results
that would shape development of the principles for development
of the standard.
Anchor
Points
An anchor
point is part of the structure of a vehicle to which a tiedown
assembly is attached. Anchor points should properly come under
federal jurisdiction as a vehicle safety standard. Transport Canada
undertook to develop a standard for designated cargo anchor points
for new vehicles, so this aspect needed no further attention.
However, the working load limit of anchor points on existing vehicles
is not known in most cases, so it is not possible for shippers,
carriers or highway inspectors to assess their real load securement
capacity. It was clear that this area needed some research.
Tiedown
Assemblies
A tiedown
assembly uses a tension device like a load binder or winch to
produce tension in a tiedown, like a chain, cable, steel strap,
webbing or rope, to secure a load to a vehicle. Current standards
are believed to assume that the tension in a tiedown anchored
at each end is uniform along its length, as if the tiedown acts
as if it passed over a pulley. This allows the tiedown to be rated
at twice its working load limit. It is evident that friction between
a tiedown and a load, and tiedowns that engage a sharp corner
or bite into a soft comer, may prevent uniform tension being developed
when the tiedown is tensioned from one side of a vehicle. If tension
in the various spans of a tiedown is not initially equal, it is
not clear whether the flexibility of a load, its tendency to move
slightly on the deck of the trailer, or its ability to change
shape, might ensure that accelerations due to roadway roughness
do in fact quickly equalize the tensions once the vehicle starts
moving. It was clearly necessary to gain a full and clear understanding
of the mechanics of tiedowns.
Blocking
BIocking is
used to immobilize a load. The load may be arranged to bear directly
or indirectly against van walls, stakes or racks. Trailer manufacturers
generally do not design van walls to carry blocking loads, and
recommend that they should not be used for this purpose. Loads
of closely-packed goods on skids that may lean against the walls
in a turn are not considered an issue, as in most cases the trailer
will roll over before a skid would tip over. Stakes and racks
use many materials and are of many designs. The project tested
typical stakes of different types in both shear and bending to
allow a working load limit to be assessed for these components.
A load may
also be blocked by timber nailed to the truck bed against the
side of the load. The project included an assessment of the realistic
levels of restraint that can be expected from typical patterns
for nailing timber blocking to a timber deck.
Dressed
Lumber
Dressed lumber
is simply a particular commodity that represents the class of
long loads which may be stacked and must be secured at intervals
along their length. This part of the project examined the capacity
and tension of tiedowns, friction between the load and the vehicle,
and whether each individual level of the load should be secured
separately.
Metal Coils
Metal coils
are one of the more difficult loads to handle and secure, due
to their weight and shape. The issues are whether blocking is
required to secure coils, whether steel bunks are required to
secure blocking, the size of the blocking, and the effectiveness
of chains in absorbing horizontal loads as a function of chain
angle, for both longitudinal and lateral eyes.
Other Issues
A number of
other potential issues arose during development of the research
proposal, beyond those identified by the Task Force report [1].
When discussion suggested that current practice and procedures
were not known to be inadequate, research was not considered necessary
at this time.
Friction between
the load and the vehicle, or between tiedowns and dunnage or the
load, is as fundamental a part of load securement as blocking,
anchor points and tiedown assemblies. It has been suggested that
friction cannot always be relied upon, so should always be discounted
[3]. However, friction was present and did play a significant
role in many of the tests, so it was necessary to determine the
magnitude and role of friction simply to interpret the results
of those tests. The findings provided an opportunity to review
the above recommendation during the regulatory development process.
It is evident
that some tension devices can develop a fixed amount of tension
in a tiedown, independent of the rating of the tiedown. This means
that a small tiedown could be a tensioned to a much greater proportion
of its capacity than a larger tiedown, which means it will have
a smaller range to absorb tension from the load itself before
failure. This raises two issues, the actual capability of tension
devices to develop tension in tiedowns, and the extent to which
more tiedowns of a lower capacity may differ from fewer tiedowns
of greater capacity, when both systems would have the same total
capacity.
It is believed
that the load rating of chain is based on a straight pull in tension.
When one link of a tensioned chain bears directly on a hard surface,
that link is subject to high stresses, and may yield. It is believed
that such factors play a part in the fact that chain has a typical
ratio between its ultimate capacity and its working load limit
of 3 or 4 to 1. However, it is also believed that this safety
margin is used by regulations to ensure reliable load securement.
While these relationships are quite unclear, it is clear that
the safety margin of chain cannot be used in both places. It was
proposed, therefore, to investigate the effect on chain strength
of the entire load being carried by one link.
Load securement
problems were identified with a number of specific commodities
carried on flatdeck trailers. The initial list was probably far
from complete, but it did at least establish an approach to commodity
specific issues.
There was
not any reason to question the manufacturers rating of chain,
webbing and cable tiedowns, load binders and D-rings, or their
standards to identify when damaged equipment should be removed
from service. The issue is to ensure that equipment of the proper
capacity is used.
The securement
of logs had already largely been addressed by industry, so was
not initially considered an issue [4, 5, 6]. However, not all
current information was widely known, so the issue re-surfaced.
After further review, it was concluded that adequate securement
techniques were available, but they needed to be better known
and more widely used. The one clear remaining issue was 1.2 m
(4 ft) pulpwood logs carried crosswise in two stacks, a current
but diminishing issue as industry moves to more efficient wood
handling practices. A special study confirmed that practices for
2.4 m (8 ft) logs would apply [7].
The securement
of heavy equipment and wheeled vehicles was being dealt with by
others at the time the proposal was being developed. Most manufacturers
consider transportation when they design their vehicles, and recommend
standards and procedures, principally to ensure the vehicle can
be delivered without damage. There is no evidence to suggest that
these procedures are inadequate when followed, so they did provide
an adequate basis to address this issue.
Conclusions
The analysis
of issues identified that a load securement system is composed
of four parts:
-
Friction,
which acts between the load and the vehicle, and may also
act between the load and other elements of the load securement
system;
-
Blocking,
which prevents movement of the load;
-
Tiedown
assemblies; and
-
Anchor
points.
These four
parts are provided in various ways, for various commodities. Devices
used for load securement may contribute to more than one of the
above four parts. Therefore, these are not independent, and may
interact in different ways, depending on the size and shape of
the load.
It was concluded
that the project should examine the mechanics of each of the above
four fundamental parts of load securement systems. It had to also
examine how they interact in common practice for specific commodities
that are known to have load securement problems.
OBJECTIVES
The research
proposal [2] defined the following three objectives:
-
To
determine how parts of load securement systems contribute to
the overall capacity of those systems;
-
To
demonstrate the adequacy of the parts, and the overall capacity,
of load securement systems; and
-
To
develop principles, based on sound engineering analysis that
would contribute to an international standard on load securement
for heavy trucks.
METHODOLOGY
If the mechanics
of a particular issue are well understood and adequate data are
available, the issue could be approached by computer simulation.
If the mechanics are not well understood, are non-linear, or data
are difficult to obtain or unreliable, then its more appropriate
to approach it by means of a test program. Many of the key mechanisms
involved in load securement, like the pulley effect, the ability
to develop tension in tiedowns, the role of friction, and the
possible domino effect failure of multiple small tiedowns, are
simply not well understood and are non-linear. It was considered
necessary to develop an understanding of the mechanics of the
parts of load securement systems, and the data needed, before
it would be possible to develop and use simple models effectively.
This means that the issues had to be addressed by a test program
[3].
Most tests,
especially those conducted in a laboratory, had to be artificial,
in the sense that the test conditions were set up to ensure that
the characteristic of interest could be observed reliably without
the confounding influence of extraneous or uncontrolled factors.
Many tests were designed to determine the capacity of load securement
systems that are beyond the maneuvering capability of a vehicle,
so the only way to determine that capacity was by a laboratory
test.
Simple load
securement models were developed for many generic combinations
of load shapes and tiedown geometry [3]. These tools were used
as necessary, to validate methods of analysis, and to move from
the specific conditions tested to the general conditions which
were necessary for development of regulatory principles.
The project
was a cooperative public-private undertaking, funded jointly by
the Canadian and U.S. federal governments, all provinces and territories,
some provincial insurance agencies, some states, Canadian and
American Trucking Associations, and some shipper associations.
It was being conducted under the guidance of a management committee
formed by CCMTA with one representative from each funding partner.
The dressed lumber portion of the work was undertaken by the Forest
Engineering Research Institute of Canada on behalf of the Ministère
des Transports du Québec. The remainder was undertaken
by Ontario Ministry of Transportation, as an in-kind contribution
to the project. A number of other agencies, associations and companies
also made specific in-kind contributions of staff, equipment and
test articles to the project. Many other similar offers were received
and declined because the necessary equipment was already available.
It is important
to recognize that the project simply conducted research. The findings
were passed on to those charged with development of a final load
securement regulation to build on the framework already developed
[1]. Regulatory development proceeded concurrently with the latter
stages of the research, as there are many organizational and procedural
matters that needed to be resolved in the pioneering process of
open development of a single uniform North America-wide standard
for load securement. There were also many matters where research
was not considered necessary and regulatory development did not
need to wait for research outcomes.
TEST PROGRAM
Anchor
Points
Load ratings
of typical anchor points are generally unknown. The proposal identified
the most common types of anchor point, like stake pockets, rub
rails, welded rods, D rings, chain-in-tube assemblies and winches.
Samples of these of different capacity were loaded in various
directions to failure in a laboratory load testing machine. Tests
also examined whether the means by which a chain is hooked to
and wrapped around a stake pocket might affect its strength as
an anchor point.
Test articles
were initially assessed by linear finite element structural analysis,
for each load case, to determine high-stress areas and likely
modes of failure. Strain gauges were installed at critical locations,
and displacement transducers were installed prior to the test.
The applied load and strain and displacement responses were monitored
and recorded by a computer-based data acquisition system, and
the load response and mode of failure was determined. Test data
was processed and compared with a non-linear finite element structural
analysis, which were used to extend the test results to a set
of principles for rating the various anchor points.
Tiedown
Assemblies
This series
of tests addressed the following issues of tiedown assemblies:
-
The
effect of binder type, chain size and chain length on the ability
to develop tension in a chain;
-
The
effect on chain strength of links bearing on hard corners;
-
Equalization
of tension in the spans of chain and webbing tiedowns;
-
The
effect of load lateral movement on chain and webbing tiedown
tension; and
-
The
effect of load longitudinal movement on chain and webbing tiedown
tension.
The first two
issues related strictly to the properties of typical tiedown assemblies.
The other three all addressed the pulley effect, as it was clearly
necessary to determine the extent to which tension equalizes in
the spans of a tiedown, for various types of load and tiedown.
Blocking
This series
of tests examined the horizontal load capacity of various configurations
of wood blocking nailed to the deck, and the shear and bending
strength of various stakes.
Wooden blocks
are often used as dunnage when chain to cable are used to secure
a load. The extent to which these tiedowns cause deformation of
the wood will have an affect on the pulley effect, as outlined
in the previous chapter.
All of the
tests in this series were done in a laboratory, to isolate the
factors of interest from confounding effects that could arise
with real loads on the highway.
Friction
This series
of tests determined the coefficients of static and sliding friction
between typical truck decks, like coarse solid hardwood, smooth
solid, sealed hardwood, smooth steel, grooved aluminum with both
longitudinal or lateral grooves, and Transdeck; typical loads,
like oak, spruce, smooth steel, machine feet, steel pads, a plastic
skid, concrete, rubber, and paper; and typical interface conditions,
like clean and dry, wet, oily and sandy. Not all interface conditions
were likely to occur for all combinations of truck deck and load,
so a selection was made to cover conditions most likely to be
found in daily practice. Tests also covered the special case of
large diameter concrete pipe.
The effect
of vibration on friction was also examined.
Dressed
Lumber
This series
of tests investigated the effect of the number and spacing of
tiedowns. For loads consisting of more than one tier of lumber,
it also investigated the difference between tiedowns over every
tier and tiedowns over all tiers. Tests were conducted using lateral
and longitudinal tilt tables, and dynamic lateral/directional
high-speed vehicle maneuvers on a test track. The load was placed
on standard wood spacers, and on Teflon-covered with spacers to
simulate the low-friction effect of an ice-covered spacer or deck.
Tests were conducted with webbing tiedowns, tightened to various
initial tensions. Instrumentation monitored load movement and
tiedown tension.
Metal Coils
This series
of tests examines the separated effects of friction, blocking
and tiedown on coils, and combined effect of all three of these
components of the load securement system.
It
included the following:
-
Effect
of friction;
-
Effect
of blocking;
-
Chain
securement: eye lateral;
-
Chain
securement. eye longitudinal;
-
Cradle
test, eye lateral. cradle secured;
-
Cradle
test, eye lateral. cradle unsecured;
-
Cradle
and chains. eye lateral, cradle unsecured.
-
Cradle
test, eye longitudinal. cradle secured;
-
Cradle
test, eye longitudinal, various securement combinations;
-
Cradle
test, eye longitudinal, combination cradle and steep angle
chains;
-
Cradle
test, eye lateral, combination cradle and shallow angle chains;
-
Overwrap
test, eye lateral and longitudinal, combined blocking and
chains: and
-
Overwrap
test, eye lateral and longitudinal, two way blocking and chains.
The size,
shape, weight, stackability and packability of the myriad of commodities
that are shipped by track each provide their own particular problems
of load securement. This series of tests examined the ability
of tiedowns to restrain a number of commodities that are recognized
to have particular problems. It addressed securement of the following
specific commodities on flatdeck vehicles: palletized loads, heavy
steel plate, large boulders, coiled wire, steel pipe, and an ISO
container.
DEVELOPMENT
OF REGULATORY PRINCIPLES
There are
two fundamental problems with research. First, it is not always
possible to address issues in a direct manner. So the relationship
between the questions originally asked and the answers finally
provided is not always clear. Second, the answers must be reported
in technical terms. It was therefore proposed to take the findings
of the research, which represent the principles of mechanics,
discuss and analyze them in the context of current industry practice
in load securement, and propose a set of principles that might
serve as a technical basis for a standard on load securement This
step intended to provide those charged with regulatory development
with the essentials necessary to draft the standard, in clear
language. The discussion proved to be quite lengthy, as there
were a considerable number of closely-related policy issues. It
also involved analysis and computer simulation, to obtain general
results from the specific conditions of tests conducted during
the work. lt was expected the discussion would identify a number
of areas where choices were available in approach, format or other
aspects of a standard. These were intended to clearly identified
and discussed, so that the consequences of choices were clear
for subsequent development of a standard. It was also expected
that certain elements of the work would identify practices, procedures
or methods that may have considerable merit, or are without merit
and should be discouraged.
The regulatory
principles were not considered to be a finished standard. First,
not all aspects of the standard were covered by this research,
so regulatory principles for the part that had been researched
needed to be blended with other current knowledge and practice.
Second, technical recommendations must often be simplified to
be used by industry and evaluated by enforcement staff.
The standard
itself is based on objective performance requirements related
to the capabilities of commercial vehicles within the confines
of the highway system. It provides prescriptive means for securement
of a significant number of categories of cargo, each meeting or
deemed to meet the performance requirements. In addition, it provides
general processes for securement of other types of cargo. These
processes allow innovation, even for those types of cargo covered
by the prescriptive clauses. The standard is written in clear
language, carefully avoiding the archaic wording and formality
of conventional regulations. It uses tables and illustrations
in whatever form is necessary to ensure the intent is clear. It
is supported by a commentary, that explains the objective, origin
and logic for each clause, and justifies the approach selected
and any numbers used. The commentary keeps the intent with the
standard, providing guidance for future maintenance of the standard.
It may also provide guidance to judges and avoid the occasional
interpretation beyond the intent. As stated previoulsy, the new
regulations on the North American Standard were slated to come
in effect in Canada on July 1st, 2005.
References:
-
CCMTA,
"New Challenges, New Opportunities, New Requirements
for Motor Carrier Load Securement", Report of Task Force
on Load Security to CCMTA Standing Committee on Compliance
and Regulatory Affairs, 1992
-
Billing,
J.R., Mercer W.R.J. & Cann W., "A Proposal for Research
to Provide a Technical Basis for a Revised National Standard
on Load Security for Heavy Trucks", Transportation Technology
and Energy Branch, Ontario Ministry of Transportation, Report
CV-93-02, 1993
-
Gillespie,
T.D., "Engineering Analysis of Cargo Restraint on Commercial
Highway Trucks", University of Michigan Transportation
Research Institute, Report UMTRI-87-28, 1987.
-
Lavoie,
J-M., "Securing Systems for Truck Loads of Tree-lengths",
Forest Engineering Research Institute of Canada, Technical
Note TN-41, 1981
-
Franklin,
G.S., "Testing Methods to Secure 8' Wood on a Tractor-Trailer
Haul Unit", Forest Engineering Research Institute
of Canada, Report TR-60, 1985
-
Franklin,
G.S., "Load Security on Tractor-Trailer Haul Units with
8-foot, 16-foot and Tree-length Wood", Forest Engineering
Research Institute of Canada, Report SR-55, 1988
-
Desrosiers,
G., "Evaluation of Common Tiedown Systems used for Securing
1.22 metre long Logs Placed Across the Bed of a Vehicle During
Transport", Ministere des Transports du Québec,
1994
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