Load Bearing Wood Walls - Live Load Reduction

[AUCE98]

IBC 2000, live load reduction is based on tributary area associated with the loaded element. In a load bearing wood wall, does the code consider the entire wall or the individual stud as the element to consider for the tributary area. My understanding is that if there is a mechanism for redistribution of load, then the wall would be considered the element to determine the tributary area. Would a double top plate be considered adequate for a load distribution element, or would the wall need to be sheathed in plywood to consider distribution to occur?

Thanks in advance for any insight.

AUCE98

 

[haynewp]

Should be the same as if you have a reinforced cmu wall, you wouldn't base the load on only one reinforced cell area since a bond beam normally distributes the load along the wall. Check the distribution element stiffness compared to the stiffness of the studs, if it is adequate, then use the reduced load.

 

[archeng59]

you are designing a stud to carry gravity (and most likely, wind) loads. in my opinion, the stud is a column and similar to a floor or roof joist in that it's part of a system. the tributary width is the stud spacing when sizing the stud. you are allowed to use a 1.15 multiplyer (per NDS) for repetitive members to increase the allowable load.

 

[kxa]

I don't have my NDS with me but if I remember correctly, the 1.15 mutipler is only used for bending (Fb)and not compression.

 

[haynewp]

You should base wind load on an individual stud ares only. But I think that live load reduction can be based on distribution to adjacent studs, the same concept as done with any other wall system.

 

[archeng59]

a stud should be checked for both compression due to vertical loads, flexure due to wind loads (or seismic) and the combined effects per section 3.9 of the NDS.

 

[haynewp]

Yeah, but that is not what he is asking.

 

[archeng59]

I may be reading his question wrong, but he's asking what tributary area to use. I say the tributary area is for a single stud, not the entire wall. the 1.15 multiplier will allow you to have a higher allowable stress and a greater load carrying capacity. similar to live load reduction, in my opinion.

 

[haynewp]

Maybe that is the way it is supposed handled in wood. I have always seen concrete and cmu walls designed for a larger trib than for just the spacing of the reinforcing. There is usually a bond beam for distribution or just the behaviour of the concrete itself distributes the vertical load to a wider area. I think that is what the original poster was implying as well.

 

[DaveAtkins]

In Section 1609.2 of the IBC, Effective Wind Area is defined as the span length multiplied by 1/3 of the span length. So when I am determining wind load on an individual stud, I use a tributary area of the span squared divided by 3.

DaveAtkins

 

[RARWOOD]

A very interesting question! If youn contact some one at the American Forest & Paper Products Association they should be able to answer your question.

The repetitive member factor of 1.15 only appies to bending stress. In general the design of wood members is based on the design of single individual members without accounting for system effects. On that basis I think I would not apply live load reduction

Also in many cases loads are transfered to the walls through closely spaced members such as floor trusses, I-joists and solid sawn joists. The load applied to these elements are going to be transfered into the supporting studs below, with some distribution occuring as a result of the top plates and wall sheathing.

Live Load reduction is based on a probability of the entire design load being present at one time. I think it is reasonable to apply that to the design of a stud wall, although I don't think you could justify by code provision.

My logic is this, the live load caused by people will occur all over the floor area. One person is standing over one floor joist and another over a different one. Also on the floor above there may be nobody standing on that joist.

In addition a distribution of load occurs through the sheathing or wood decking when closely space members deflect. The repetitive member factor is based on this distribution.

One final comment there are some new publications available from AFPA which due account for system effect. Accounting for systems effects may be more useful than applying live load reduction.

 

[archeng59]

I'd like to clarify, or correct if necessary, my proposed use of the 1.15 multiplier. I agree it is used only for bending stress. when designing studs, I check both axial compression and bending. I believe the code requires a minimum wind pressure of 5 psf for all load bearing members, in this case a stud. when calculating the allowable bending stress, use the 1.15 multiplier. when checking combined stresses, I use the allowable bending stress which includes the 1.15 multiplier. is that an incorrect application of the multiplier?

using the multiplier when checking combined stresses will show a stud to be adequate when it may not be so if the stresses are checked without the multiplier.

 

[Taro]

I recommend that you be VERY careful when considering a live load reduction for stud walls. The tributary area for an entire wall that is carrying two or more floors could easily be enough to reduce the live load to 40% of the full value. But the tributary area for a single stud is small enough that it could very well see the full live load. It is very unlikely that the load distributing elements (top plate, etc.) have enough flexural stiffness in comparison to the axial stiffness of the stud to transfer 60% of the load.

Note that IBC and ASCE-7 both restrict live load reductions for one-way slabs. Stud walls would seem to behave similarly to one-way slabs.

 

[RARWOOD]

archeng59

I think you can apply the repetitive member adjustment to the bending stress whenever you meet the requirement of 4.3.4 of the 1997 N.D.S. (4.3.9 2001 N.D.S.). The N.D.S.indicates that the repetitive adjustment applies to studs.

It is my understanding that in the combine stress equations you determine your allowable bending stress based on what would be allowed if you had bending only.

One thing you might look into is designing using LRFD. I am not familair with LRFD but it is my understanding that some of the confusion in the past regarding some adjustment factors has been cleared up.

 

[haynewp]

I think Taro is right. The double plate is not going to have the relative stiffness you need to distribute the load like in other construction.

 

[archeng59]

thanks, RARSWC. I've been using the multiplier in that manner for years, but wondered if I might be mis-using it.

 

[SlideRuleEra]

IBC 2000 (and IBC 2003) adopt NDS for the most part, but not totally. For wind loading on walls with studs at 16" oc that have gypsum board on one side and wood structural panels on the other, IBC supplies a table of values that supersede the repetitive member factor (the IBC factors are GREATER than 1.15).

See paragraph 2306.2.1 in both IBC 2000 and IBC 2003 for the details.

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