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1
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Mike Mike
Structural
- Apr 27, 2019
- 136
IBC 2018 1607.10.4
"Fall arrest and lifeline anchorages. In addition to
any other applicable live loads, fall arrest and lifeline anchor-ages
and structural elements that support these anchorages
shall be designed for a live load of not less than 3,100 pounds
(13.8 kN) for each attached lifeline, in every direction that a
fall arrest load can be applied."
IBC 2018 1607.10.4 Commentary
"Lifeline anchorages, also known as fall arrest anchorages,
are called on to resist impact loads when a suspended
worker on the face of a building experiences
a fall. Because the loads are highly variable depending
on the weight of the worker, the fall distance and
the energy-absorbing characteristics of the fall arrest
system, and because the lifeline is the last defense
against a fall, OSHA requires that lifeline anchorages
be capable of sustaining without failure an ultimate
load of 5,000 pounds (22.2 kN) per person. Using a
design live load of 3,100 pounds (13.8 kN), when
combined with a live load factor of 1.6, results in a
total factored load of 4,960 pounds (22.1 kN), which
matches OSHA’s requirements for lifeline anchorages
within an acceptable margin of error. The load is
used to design the lifeline anchorage and the structural
elements that support the anchorage."
OSHA 1926.502
"(d)(15) Anchorages used for attachment of personal fall arrest equipment shall be independent of any anchorage being used to support or suspend platforms and capable of supporting at least 5,000 pounds (22.2 kN) per employee attached, or shall be designed, installed, and used as follows:
(d)(15)(i) as part of a complete personal fall arrest system which maintains a safety factor of at least two; and
(d)(15)(ii) under the supervision of a qualified person."
It's good to see ICC jumping into the fall arrest confusion and trying to clear this up so structural engineers are provided with clear guidance. However, some might say ICC failed to do their research. Two issues with 1607.10.4:
1. Nobody knows where OSHA's 5,000 pound requirement came from. One theory proposes this was the ultimate strength of hemp rope originally used in fall protection (post your favorite theory here!!!). OSHA and fall protection are not my area of expertise, but I know OSHA does limit loads applied to human beings to 1,800 lbs. The 1,800lb lanyard maximum arresting force (MAF) makes more sense than the arcane 5,000 lbs as a basis for structural design.
2. The word "ultimate" unfortunately has two (or possibly more than two) meanings in structural engineering. It's used to refer to both the combined LRFD load, and the strength determined by testing. For example, Simpson says their 3/8" Titen HDs are tested to an ultimate load of 2,390lbs in CMU, but the allowable load is only 480lbs for use in ASD design (safety factor of 5). The 2,390lb ultimate load has nothing to do with LRFD, and I would argue OSHA's 5,000lb ultimate load has nothing to do with LRFD either. It seems ICC has misinterpreted OSHA's use of the word to mean LRFD factored load, and has assumed the controlling load combination is 1.2D + 1.6L + 0.5(Lr or S or R).
I'll throw a specific ASD example out to kick things off: An A36 bolted plate connection in simple tension controlled by the limit state of AISC 2016 J4.1(b) - tensile rupture. AISC specifies a safety factor, omega, of 2. One might say the intent of OSHA's "safety factor of at least two" has already been met within AISC's prescription. No additional safety factor would need to be applied on the load side, and the connection would be designed for an ASD load of 1,800lbs. In addition, one might argue the design load should be even lower than 1,800lbs because there is an additional safety factor rolled into the specified minimum tensile strength, Fu, since the expected tensile strength per AISC341-10 A3.2 is actually 20% higher than the 58ksi minimum per AISC manual table 2-5. IBC disagrees, and prescribes a 3,100lb ASD load.
Fall arrest anchorage should be designed the way structural engineers design everything else. Namely, determine how strong the connection actually needs to be, then apply the factor of safety. Factors of safety are not meant to be applied at each step along the design process, on both the load side and the capacity side of the equations, in the haphazard hopes that if loads are rounded up and capacities are rounded down enough times we should hopefully be safe enough. Researchers and code committees generally find the number of standard deviations between the actual capacity and the actual demand mean values, or beta factor, should be about 3 to properly balance safety and economy. One might argue ICC should disregard the bureaucratic, cumbersome OSHA and base their code requirements on the reason so that the construction industry has some consensus and structural engineers don't need to go digging through the mud for explanations.
"Fall arrest and lifeline anchorages. In addition to
any other applicable live loads, fall arrest and lifeline anchor-ages
and structural elements that support these anchorages
shall be designed for a live load of not less than 3,100 pounds
(13.8 kN) for each attached lifeline, in every direction that a
fall arrest load can be applied."
IBC 2018 1607.10.4 Commentary
"Lifeline anchorages, also known as fall arrest anchorages,
are called on to resist impact loads when a suspended
worker on the face of a building experiences
a fall. Because the loads are highly variable depending
on the weight of the worker, the fall distance and
the energy-absorbing characteristics of the fall arrest
system, and because the lifeline is the last defense
against a fall, OSHA requires that lifeline anchorages
be capable of sustaining without failure an ultimate
load of 5,000 pounds (22.2 kN) per person. Using a
design live load of 3,100 pounds (13.8 kN), when
combined with a live load factor of 1.6, results in a
total factored load of 4,960 pounds (22.1 kN), which
matches OSHA’s requirements for lifeline anchorages
within an acceptable margin of error. The load is
used to design the lifeline anchorage and the structural
elements that support the anchorage."
OSHA 1926.502
"(d)(15) Anchorages used for attachment of personal fall arrest equipment shall be independent of any anchorage being used to support or suspend platforms and capable of supporting at least 5,000 pounds (22.2 kN) per employee attached, or shall be designed, installed, and used as follows:
(d)(15)(i) as part of a complete personal fall arrest system which maintains a safety factor of at least two; and
(d)(15)(ii) under the supervision of a qualified person."
It's good to see ICC jumping into the fall arrest confusion and trying to clear this up so structural engineers are provided with clear guidance. However, some might say ICC failed to do their research. Two issues with 1607.10.4:
1. Nobody knows where OSHA's 5,000 pound requirement came from. One theory proposes this was the ultimate strength of hemp rope originally used in fall protection (post your favorite theory here!!!). OSHA and fall protection are not my area of expertise, but I know OSHA does limit loads applied to human beings to 1,800 lbs. The 1,800lb lanyard maximum arresting force (MAF) makes more sense than the arcane 5,000 lbs as a basis for structural design.
2. The word "ultimate" unfortunately has two (or possibly more than two) meanings in structural engineering. It's used to refer to both the combined LRFD load, and the strength determined by testing. For example, Simpson says their 3/8" Titen HDs are tested to an ultimate load of 2,390lbs in CMU, but the allowable load is only 480lbs for use in ASD design (safety factor of 5). The 2,390lb ultimate load has nothing to do with LRFD, and I would argue OSHA's 5,000lb ultimate load has nothing to do with LRFD either. It seems ICC has misinterpreted OSHA's use of the word to mean LRFD factored load, and has assumed the controlling load combination is 1.2D + 1.6L + 0.5(Lr or S or R).
I'll throw a specific ASD example out to kick things off: An A36 bolted plate connection in simple tension controlled by the limit state of AISC 2016 J4.1(b) - tensile rupture. AISC specifies a safety factor, omega, of 2. One might say the intent of OSHA's "safety factor of at least two" has already been met within AISC's prescription. No additional safety factor would need to be applied on the load side, and the connection would be designed for an ASD load of 1,800lbs. In addition, one might argue the design load should be even lower than 1,800lbs because there is an additional safety factor rolled into the specified minimum tensile strength, Fu, since the expected tensile strength per AISC341-10 A3.2 is actually 20% higher than the 58ksi minimum per AISC manual table 2-5. IBC disagrees, and prescribes a 3,100lb ASD load.
Fall arrest anchorage should be designed the way structural engineers design everything else. Namely, determine how strong the connection actually needs to be, then apply the factor of safety. Factors of safety are not meant to be applied at each step along the design process, on both the load side and the capacity side of the equations, in the haphazard hopes that if loads are rounded up and capacities are rounded down enough times we should hopefully be safe enough. Researchers and code committees generally find the number of standard deviations between the actual capacity and the actual demand mean values, or beta factor, should be about 3 to properly balance safety and economy. One might argue ICC should disregard the bureaucratic, cumbersome OSHA and base their code requirements on the reason so that the construction industry has some consensus and structural engineers don't need to go digging through the mud for explanations.
