ETL 1110-2-534
30 Sep 94
cracks. High stress areas, such as at railing and post
capable of carrying, with little or no permanent defor-
support points, should be given careful inspection.
mation, loads much greater than those computed on
Members that are susceptible to debris and water
the basis of the conservative conventional bending
collection, such as at the base of posts, should be
stresses. With these facts in mind, the engineer must
inspected internally for corrosion. If section loss
make a rational decision as to the allowable bending
(internal or external) has occurred, the remaining
stress. For USACE guardrails that are located in
section should be carefully measured and those values
areas of high public access, such as daily guided tour
used in the structural analyses. If cracking is
stops, the conservative and conventional allowable
observed, those particular members should be
stresses provided in Table A-1 should be used. For
immediately repaired or replaced.
guardrails in low traffic areas, where railings are
rarely leaned upon, an allowable stress up to 0.85fy
(2) The condition of the guardrail's anchorage to
may be used at the engineer's discretion. However,
its supporting platform must also be checked. Verify
due to the often detrimental effects of welding alumi-
that anchor bolts are all still in place, tightly
num, only the allowable stresses shown in Table A-1
anchored, and in good condition. If welded, check
should be used for welded aluminum railing, regard-
the welds for cracks and deterioration. Welded alum-
less of the railing usage.
inum is particularly susceptible to cracking caused
from differential expansion and contraction between
d.
Sectional properties.
dissimilar metals.
(1) Pipe is produced in a variety of sizes or
c.
Material properties.
"schedules," of which those more commonly used for
railings are listed in Table A-2. The sections shown
(1) Guardrails are constructed from a wide range
in Table A-2 are by no means all-inclusive. Many
of metals. Mechanical properties and allowable
manufacturers have proprietary sections that have
design stresses of some of the more commonly used
unique properties. Information on these sections can
metals are provided in Table A-1. Carefully note that
often be obtained from supplier's catalogs or from
the allowable tensile stresses for tempered (T-type)
industry associations. In steel pipe, Schedule 40 is
know as "Standard Weight" and Schedule 80 as
aluminum alloys must be reduced at all locations
"Extra Strong." Standard weight is measured by
within 25 mm (1 in.) of welds.
i.p.s. (iron pipe size), which designates its nominal
size. Unless otherwise specified, Schedule 40 is
(2) The allowable stresses shown in Table A-1
normally supplied in steel, aluminum, or copper pipe
are based on recommendations from NAAMM (1985)
and Schedule 5 in stainless steel tubing.
and NOMMA (1986). The NAAMM manual notes
that some designers feel that, particularly in the case
(2) Round tubing is also available in all four
of high concentrated loads such as the 890-N (200-lb)
metals. Tubing differs from pipe in that it may have
load specified by the USACE, the use of higher
thinner or thicker walls and is measured by a differ-
allowable stresses may be justified. This is based on
ent system, which designates the outside diameter and
the fact that this type of loading will be of a momen-
the wall thickness. The wall thickness is designated
tary rather than sustained nature. The validity of this
in decimal inches or gauge number. Size designa-
position largely depends upon what is considered
tions may differ somewhat with the different metal.
acceptable railing performance and how "failure" is
defined. The purpose of safety regulations is to
(3) For structural analyses of existing railing, the
ensure that the railing provides protection against per-
section properties of the pipe must be carefully
sons falling, and the railing need not remain in per-
obtained. This can be accomplished through use of
fect alignment to perform this function. If a slight
reliable as-built records on the project or through
permanent deformation under this conservative load-
careful field measurements. As-built records, backed
ing is deemed acceptable, this would theoretically
up by field measurements, are the most desirable
permit the use of an allowable bending stress
combination since accurate section properties are
approaching the yield stress. Of course, the use of
essential to an accurate structural analysis. Accurate
yield stress for the allowable stress is not easily palat-
field measurements are also important since as-built
able to engineers, who must always be conservative.
records will only list "nominal" sizes. The ASTM
However, actual physical testing has repeatedly dem-
standards for pipe and tubing extrusions provide
onstrated that securely anchored pipe railings are
A-2
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