girt deflection limits

[OzarkMTBr]

I am curious as to what some of you hold the deflection to an exterior wall girt using the IBC2000 code if the girt was supporting a brittle masonry wall?

Also, those familiar with the SBC97, are you allowed to reduce your wind load on said girt above by 0.75?

Thanks

 

[dik]

Pre eng type buildings with girts often have their own 'standards' that may be less than other requirements.

I'd spec/design for a maximum deflection of about L/600 or L/720 or whatever the SBC or IBC requires for masonry...

 

[JCWilson]

OzarkMTBr,

Definitely use at least L/600 as the deflection limit for a masonry wall. Brick Industry Association Technical Note #28b (

http://www.brickinfo.org/pdfs/28b.pdf)

discusses brick deflection limits for a steel stud supporting wall. I would think the same logic would apply to your case. That is the best definitive reference I have been able to find on the web after a lot of searching.

I am not familiar with the SBC, but the allowable wind load reduction (if still valid) should only be applicable to stress calcs, not deflection. However, given that most general building codes still had an allowable 75% wind load factor ca 1996/97, the SBC may also allow it, provided that code governs for your project.

 

[OzarkMTBr]

Thanks for your input and searching. I was not aware of the technical notes on-line that you provided.
The SBC does allow for the 75% reduction of wind load. I guess my question was misleading. I was really needing to know if that could be applied to the deflection too. Some of the Code requirments seem to be vague and can be interpretted different ways. I am an EIT and have caught PE's who were interpetting the particular code on that project wrong. They corrected their way but it made me realize how easy it is to do. I am sure I am doing it on other areas of the code.

Thanks again.

 

[jbanderson]

Typical wall girt deflection in Pre-Engineered Buildings is L/240, for girts supporting brittle material.

 

[MikeE55]

I recommend you refer to a booklet published by AISC, "Serviceability Design Considerations for Low-Rise Buildings". You may be suprised to learn they recommend a deflection limit of only L/240, provided the wall can "crack" at the floor line or bottom support. The IBC 2000 does not permit the .75 reduction, but if you are designing to SBC, of course it would be permitted. It is also permitted for the deflection, but this changes the probability of the wind from a 50 year design to a 10 year design. (This is also referred to as acceptable in the AISC booklet.)

 

[SmithJ]

Hi MikeE55,

Section 1605.3.2 of the IBC 2000 states that " ...combinations that include wind or seismic loads, allowable stresses are permitted to be increased or load combinations reduced, ..."

I take that to mean that IBC does permit the .75 reduction contrary to what you stated in your reply. Please help me clarify your assertion, it's possible that I may be interpreting the code the wrong way.

For Masonry Deflection limits, I typically refer to Table 1617.3 of IBC 2000 and use that as a guideline to determine allowable girt deflection. These requirements are usually more strict even than the L/240 requirement.

JS.

 

[MikeE55]

There are actually two load combination sets of formulas, the basic combinations shown in 1605.3.1 and the alternate combinations shown in 1605.3.2. 1605.3.1.1 states that the .75 factor can be used only where there are two transient loads - in other words, since the girt is designed for only one transient load, wind, the .75 reduction is not allowed. The 1605.3.2 combinations of which you refer do permit the .75 load reduction factor. However, so that there is not an unfair advantage over the basic combinations, when you use the alternate combinations, you must multiply the wind by 1.3. Since 1.3 x .75 = 1.0, the reduction is effectively gone.

Table 1617.3 is for limiting seismic drift. I don't think it is appropriate for a serviceability check. Generally the Codes don't address serviceability.

 

[OzarkMTBr]

Thanks for you input,

MikeE55, I am currently designing wind to a girt with .75x for deflection and 1.0x for stress. I have been designing wind girts to L/360 in the past because a brick manufacturer suggested that to resist cracking. Above some of the replys to my initial question recommend L/720 for girts supporting masonry or brittle finishes. That seems excessive and not cost effective. I am trying to find out what everyone else is using for wind girts supporting masonry. I didn't know that the L/240 was supposed to work with a crack at the base for rotation I assume. The brick manufacturer I had talked with said that L/240 was not enough to prevent cracks.

Thanks for you replys

 

[OzarkMTBr]

Thanks for your input,

MikeE55, I am currently designing wind to a girt with .75x for deflection and 1.0x for stress. I have been designing wind girts to L/360 in the past because a brick manufacturer suggested that to resist cracking. Above some of the replys to my initial question recommend L/720 for girts supporting masonry or brittle finishes. That seems excessive and not cost effective. I am trying to find out what everyone else is using for wind girts supporting masonry. I didn't know that the L/240 was supposed to work with a crack at the base for rotation I assume. The brick manufacturer I had talked with said that L/240 was not enough to prevent cracks.

Thanks for you replys

 

[OzarkMTBr]

In the IBC2000 Table 1604.3 subscript 'f' reads:

f. The wind load is permitted to be taken as 0.7 times the "component and cladding" loads for the purpose of determining deflection limits herein.

Would a girt be considered a 'component'? If it can be, then frames would be designed to 0.75x and girts to 0.7x

 

[MikeE55]

The girt is definitely a component. I think what the IBC is doing in subscript "f" is allowing you to change the basis of the design from a 50 year wind to a 10 year wind. The 10 year wind is approximately 70 to 75 percent of the 50 year event. It was very convenient in the past to talk about the 10 year wind since you were allowed to reduce your analysis for stress by .75.

Anyone else have any thoughts on this?