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NYC Building Code Deflection Criteria for Steel Structural Member
- Thread starter akiddo
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Look at Table 1604.3 on
It could make sense. For concrete member, dead load plays a significant role in creating deflection because not only there is heavier self weight but also concrete deflect over a longer period of time. Therefore, one must consider dead load absolutely for concrete member. But for steel, partitions are installed after the first immediate deflection. Therefore, it might be safer to say dead load might not cause any impact on partitions for steel structures.
My questions come when we are talking about light concrete topping or composite slab.
It could make sense. For concrete member, dead load plays a significant role in creating deflection because not only there is heavier self weight but also concrete deflect over a longer period of time. Therefore, one must consider dead load absolutely for concrete member. But for steel, partitions are installed after the first immediate deflection. Therefore, it might be safer to say dead load might not cause any impact on partitions for steel structures.
My questions come when we are talking about light concrete topping or composite slab.
Table 1604.3 has a deflection limit of l/360 for L and l/240 for L + D for Roof members supporting plaster ceiling or for Floor members. If D is taken as zero, what is the point of stating the limit for L + D? It makes no sense.
However, Article 1604.3.3 Steel requires that the deflection of steel members must comply with one or more of a variety of codes which presumably take into account dead load deflection.
Partitions are one consideration for limiting deflections but not the only one. Most codes provide different limits for Live load vs. Live + Dead Load. A steel beam could be carrying substantial dead load, so it cannot be ignored when calculating combined deflection.
BA
However, Article 1604.3.3 Steel requires that the deflection of steel members must comply with one or more of a variety of codes which presumably take into account dead load deflection.
akiddo said:
Partitions are one consideration for limiting deflections but not the only one. Most codes provide different limits for Live load vs. Live + Dead Load. A steel beam could be carrying substantial dead load, so it cannot be ignored when calculating combined deflection.
BA
The IBC has the same requirement. In the 2015 IBC, footnote d was revised to clarify the intent: "The deflection limit for the D+L load combination only applies to the deflection due to the creep component of the long-term dead load deflection plus the short-term live load deflection. . . " (
Dead load on steel elements can be taken as 0 because steel doesn't creep.
Dead load on steel elements can be taken as 0 because steel doesn't creep.
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@BAretired
Look at note 'g' on the reference table I mentioned earlier. It clearly says dead load shall be taken as zero.
@wannabeSE
So, how would you treat a floor that has steel decking with concrete topping supported by steel elements (both types non-composite and composite)? Are you still going to ignore dead load for deflection?
Look at note 'g' on the reference table I mentioned earlier. It clearly says dead load shall be taken as zero.
@wannabeSE
So, how would you treat a floor that has steel decking with concrete topping supported by steel elements (both types non-composite and composite)? Are you still going to ignore dead load for deflection?
pasted below from the site of SK Gosh:
"A: Sometimes we can find the best answer to a code-related question by doing a historical check on the introduction of the provision into the code. Our check revealed that Footnote g to Table 1604.3 was approved for inclusion in the 2000 IBC (it was Footnote f back then), based upon its historical acceptance in legacy codes – the NBC, SBC and UBC. Here is the code change to prove it!
D = 0 for steel structures has no other strictly technical justification that we can find. The D=0 footnote creates two deflection limits in Table 1604.3 for steel floor members: 1) l/360 by the "L" column, and 2) l/240 by the "D + L" column. Because of the following sentence in 2012 IBC Section 102.1: "Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern," the more restrictive deflection limit of l/360 applies."
there's also this previous thread: Serviceability Requirements under IBC 2006
"A: Sometimes we can find the best answer to a code-related question by doing a historical check on the introduction of the provision into the code. Our check revealed that Footnote g to Table 1604.3 was approved for inclusion in the 2000 IBC (it was Footnote f back then), based upon its historical acceptance in legacy codes – the NBC, SBC and UBC. Here is the code change to prove it!
D = 0 for steel structures has no other strictly technical justification that we can find. The D=0 footnote creates two deflection limits in Table 1604.3 for steel floor members: 1) l/360 by the "L" column, and 2) l/240 by the "D + L" column. Because of the following sentence in 2012 IBC Section 102.1: "Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern," the more restrictive deflection limit of l/360 applies."
there's also this previous thread: Serviceability Requirements under IBC 2006
I do not agree with Hank Martin of the American Iron and Steel Institute when he claims that Footnote "f" (currently Footnote "g") will bring the IBC back into conformance with current practice. So far as I am aware, current practice is to limit deflection of a steel beam to l/360 for L and l/240 for D + L.
A deflection of l/240 for D + L is not necessarily a more stringent criteria than l/360 for L; it is only more stringent when D > L/2.
BA
A deflection of l/240 for D + L is not necessarily a more stringent criteria than l/360 for L; it is only more stringent when D > L/2.
BA
Would echo what wannabeSE notes above and think it's important to keep in mind what the deflection limit is actually going for.
Seems to me the portion of D they're really going for with the L/240 limit is the portion that occurs long-term and/or after attachment of nonstructural elements. AISC 360 notes this in commentary to Chapter L. ACI notes this in their deflection limit table (table 9.5(b) in 318-08) and notably does NOT include full dead load in any of their limits in that table. You get to use a reduced dead load in wood if your moisture content is low enough so creep isn't as large a concern (footnote d in IBC 2009). None of these would make any sense if you're trying to limit deflection for the full dead load.
To respond to akiddo's last hypothetical, AISC 360-10 commentary to Chapter L - Design for Serviceability specifically notes that the concrete would not be included:
"The dead load effect, D, may be that portion of dead load the occurs following attachment of nonstructural elements. For example, in composite construction, the dead load effects frequently are taken as those imparted after the concrete has cured. For ceiling related calculations, the dead load effects may include only those loads placed after the ceiling structure is in place."
At least for most steel structures I do, the 'dead load' that occurs after attachment of nonstructural elements is either nothing or close to nothing as steel doesn't experience significant creep and the nonstructural elements are usually the last dead load installed.
Seems to me the portion of D they're really going for with the L/240 limit is the portion that occurs long-term and/or after attachment of nonstructural elements. AISC 360 notes this in commentary to Chapter L. ACI notes this in their deflection limit table (table 9.5(b) in 318-08) and notably does NOT include full dead load in any of their limits in that table. You get to use a reduced dead load in wood if your moisture content is low enough so creep isn't as large a concern (footnote d in IBC 2009). None of these would make any sense if you're trying to limit deflection for the full dead load.
To respond to akiddo's last hypothetical, AISC 360-10 commentary to Chapter L - Design for Serviceability specifically notes that the concrete would not be included:
"The dead load effect, D, may be that portion of dead load the occurs following attachment of nonstructural elements. For example, in composite construction, the dead load effects frequently are taken as those imparted after the concrete has cured. For ceiling related calculations, the dead load effects may include only those loads placed after the ceiling structure is in place."
At least for most steel structures I do, the 'dead load' that occurs after attachment of nonstructural elements is either nothing or close to nothing as steel doesn't experience significant creep and the nonstructural elements are usually the last dead load installed.
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