chrislsnider
Structural
- Sep 5, 2012
- 27
It appears the consensus (thread 1, Thread 2 and conversations I've had with many other engineers) is to use L/600 for horizontal deflection of studs backing masonry veneer. It's what I've used for my entire career. Where did that come from? I do understand that UFC 3-301-01 has adopted it into their standards, but let's talk about non-military jobs.
The brick Industry Technical Notes 28B - Brick Veneer and Steel Stud Walls says:
[ul]
[li]Steel Studs Deflection Criteria
Steel studs must be designed to provide adequate out-of plane support for all loads imposed on the wall system. This is done by establishing a maximum deflection limit on the stud while maintaining steel stress values in the stud within permissible limits. This deflection is calculated assuming the entire out-of-plane load is resisted by the studs alone, neglecting contribution of the brick veneer. While a number of design tables are based on a stud deflection of stud span length divided by 360 (L/360), using this criterion may permit more deflection than the veneer is able to tolerate. Therefore, to obtain sufficient backing stiffness, the allowable out-of-plane deflection of the studs due to service level loads should be restricted to L/600. Such deflection criterion will allow a maximum crack width of about 0.015 inches (0.38 mm) in the brick veneer wythe for typical floor-to-floor dimensions.[/li]
[/ul]
I found a "design Guide for anchored brick veneer over steel stud systems" that says:
[ul]
[li]Currently, there is no generally accepted single design and construction procedure for brick veneer with a steel stud backing. A recent article warns of an unacceptable risk of failure [1]. Conflicting opinions state that the evolution of the BV/SS system is largely correcting the early problems [2], [3] & [4]. Yet, industry design recommendations are not always consistent. footnote -> An example is the design deflection limitation on the backing wall. Metal stud manufacturers recommend L/360. The Brick Industry Association recommends L/600 [/li]
[li]There are two levels of performance presented.The basis for distinguishing between levels is the anticipated system life. The first level, Level 1, is for institutional buildings with a long design life. The second level, Level 2, is for commercial buildings with an average design life. [/li]
[li]In the absence of extra ordinary emphasis on performance, the deflection limit of L/360 is recommended. This limitation was chosen to limit brick veneer crack widths to between 0.02 to 0.04 inches (0.50mm to 1.0mm) [8]. This relationship is arrived at strictly from a geometric perspective. A deflection limit of L/360 for 3 5/8 inch thick brick will result in an outside crack width of 0.04028 inches. This crack width will be smaller with decreasing brick thickness and correspondingly larger with increasing brick thickness. The crack width at the center of the brick veneer will be one-half these values. This is illustrated in Figure 2.1.[/li]
[/ul]
This article from Structure Magazine - May 2008 (granted this was for wood stud research, but he answer applies the same):
[ul]
[li]Is L/600 too stringent for out-of-plane bending for a serviceability issue? Canadian research titled Technics Steel Stud / Brick Veneer Walls, by Trestain and Rousseau drew from earlier McMaster University studies. The McMaster studies actually constructed veneer stud walls and tested with wind pressure and simulated rain.
The result was that there was no increased system vulnerability due to excessive leakage from flexural cracking. The L/720, L/600, or L/360 deflection limits do not eliminate flexural cracking. The deflection limit is intended to reduce the flexural cracking size. But as the McMaster study indicated, the size of the flexural cracking did not increase system vulnerability.[/li]
[/ul]
Table 1604.3 of the IBC ('09) says exterior walls with brittle finishes should be L/240.
ACI 530 / MSJC states
[ul]
[li]6.2.1.b - Out-of-plane deflections of the backing shall be limited to maintain veneer sability.[/li]
[li]Vertical deflection is expressly addressed in 6.2.2.3.1.5 -- the L/600 or 3/8" that we are all used to seeing.[/li]
and in the commentary:
[li]The Metal Lath/Steel Framing Association has promoted a deflection limit of stud span length divided by 3606.4. The Brick Industry Association has held that an appropriate deflection limit should be in the range of stud span length divided by 600 to 720. The deflection is computed assuming that all of the load is resisted by the studs. Neither set of assumptions will necessarily ensure that the veneer remains uncracked at service load. In fact, the probability of cracking may be high. However, postcracking performance is satisfactory if the wall is properly designed, constructed and maintained with appropriate materials. [/li]
[/ul]
So, all of this to say that it appears that there is not a code requirement for L/600 design, correct?. If we consider the Level 1/2 buildings as described in the "Design Guide..." it would be reasonable that the typical office building-type project could realize some savings from using L/360 with the .7 reduction allowed by IBC. However, a municipal building-type project should use the L/600 based off of the expected lifespan of the building. In no cases that I've come across are the architects expecting the masonry to be the moisture barrier to the building, so allowing larger cracks (that are going to happen anyway) may be acceptable.
If you've made it this far into my post/question, please don't just respond with "we always use L/___ for designs". I did to. I'm looking for some backup or reasoning as to why you would use, say, L/360 instead of L/600.
Thanks
The brick Industry Technical Notes 28B - Brick Veneer and Steel Stud Walls says:
[ul]
[li]Steel Studs Deflection Criteria
Steel studs must be designed to provide adequate out-of plane support for all loads imposed on the wall system. This is done by establishing a maximum deflection limit on the stud while maintaining steel stress values in the stud within permissible limits. This deflection is calculated assuming the entire out-of-plane load is resisted by the studs alone, neglecting contribution of the brick veneer. While a number of design tables are based on a stud deflection of stud span length divided by 360 (L/360), using this criterion may permit more deflection than the veneer is able to tolerate. Therefore, to obtain sufficient backing stiffness, the allowable out-of-plane deflection of the studs due to service level loads should be restricted to L/600. Such deflection criterion will allow a maximum crack width of about 0.015 inches (0.38 mm) in the brick veneer wythe for typical floor-to-floor dimensions.[/li]
[/ul]
I found a "design Guide for anchored brick veneer over steel stud systems" that says:
[ul]
[li]Currently, there is no generally accepted single design and construction procedure for brick veneer with a steel stud backing. A recent article warns of an unacceptable risk of failure [1]. Conflicting opinions state that the evolution of the BV/SS system is largely correcting the early problems [2], [3] & [4]. Yet, industry design recommendations are not always consistent. footnote -> An example is the design deflection limitation on the backing wall. Metal stud manufacturers recommend L/360. The Brick Industry Association recommends L/600 [/li]
[li]There are two levels of performance presented.The basis for distinguishing between levels is the anticipated system life. The first level, Level 1, is for institutional buildings with a long design life. The second level, Level 2, is for commercial buildings with an average design life. [/li]
[li]In the absence of extra ordinary emphasis on performance, the deflection limit of L/360 is recommended. This limitation was chosen to limit brick veneer crack widths to between 0.02 to 0.04 inches (0.50mm to 1.0mm) [8]. This relationship is arrived at strictly from a geometric perspective. A deflection limit of L/360 for 3 5/8 inch thick brick will result in an outside crack width of 0.04028 inches. This crack width will be smaller with decreasing brick thickness and correspondingly larger with increasing brick thickness. The crack width at the center of the brick veneer will be one-half these values. This is illustrated in Figure 2.1.[/li]
[/ul]
This article from Structure Magazine - May 2008 (granted this was for wood stud research, but he answer applies the same):
[ul]
[li]Is L/600 too stringent for out-of-plane bending for a serviceability issue? Canadian research titled Technics Steel Stud / Brick Veneer Walls, by Trestain and Rousseau drew from earlier McMaster University studies. The McMaster studies actually constructed veneer stud walls and tested with wind pressure and simulated rain.
The result was that there was no increased system vulnerability due to excessive leakage from flexural cracking. The L/720, L/600, or L/360 deflection limits do not eliminate flexural cracking. The deflection limit is intended to reduce the flexural cracking size. But as the McMaster study indicated, the size of the flexural cracking did not increase system vulnerability.[/li]
[/ul]
Table 1604.3 of the IBC ('09) says exterior walls with brittle finishes should be L/240.
ACI 530 / MSJC states
[ul]
[li]6.2.1.b - Out-of-plane deflections of the backing shall be limited to maintain veneer sability.[/li]
[li]Vertical deflection is expressly addressed in 6.2.2.3.1.5 -- the L/600 or 3/8" that we are all used to seeing.[/li]
and in the commentary:
[li]The Metal Lath/Steel Framing Association has promoted a deflection limit of stud span length divided by 3606.4. The Brick Industry Association has held that an appropriate deflection limit should be in the range of stud span length divided by 600 to 720. The deflection is computed assuming that all of the load is resisted by the studs. Neither set of assumptions will necessarily ensure that the veneer remains uncracked at service load. In fact, the probability of cracking may be high. However, postcracking performance is satisfactory if the wall is properly designed, constructed and maintained with appropriate materials. [/li]
[/ul]
So, all of this to say that it appears that there is not a code requirement for L/600 design, correct?. If we consider the Level 1/2 buildings as described in the "Design Guide..." it would be reasonable that the typical office building-type project could realize some savings from using L/360 with the .7 reduction allowed by IBC. However, a municipal building-type project should use the L/600 based off of the expected lifespan of the building. In no cases that I've come across are the architects expecting the masonry to be the moisture barrier to the building, so allowing larger cracks (that are going to happen anyway) may be acceptable.
If you've made it this far into my post/question, please don't just respond with "we always use L/___ for designs". I did to. I'm looking for some backup or reasoning as to why you would use, say, L/360 instead of L/600.
Thanks
