1997 UBC Collector Element Design 2

[whyun]

In the 1997 UBC, we are asked to use the special load combination (which is at omega level force) for a column supporting a discontinuous lateral system (vertical irregularity) using a allowable stress increase of 1.7 times the load duration factor (1.33 for seismic).

Similar requirement exists for transfer diaphragms.

Collector members also have a similar requirement.

My question: In Section 1633.2.6, special seismic combination requirement is exempt for wood framed structures. Basically, just design the collector using the force from Fp equation then compare to the allowable load times 1.33 load duration. Is there is a particular reason behind this exemption? Any backgroud info would be most helpful.

 

[JAE]

whyun,

Not sure the "why" behind the provision. I know that there are ICBO documents that record the debates and conclusions for various code items.

I first thought of this: The Exception in 1633.2.6 refers to "light frame" construction. That is an explicit term in the UBC that I thought refered to Division IV of Chapter 23, "Conventional Light Frame Construction". This is an empirical design procedure that can govern wood framed buildings such as 1, 2, and 3 story apartment buildings. However, after thinking through it...why would you use Division IV and even need to go to chapter 16? Didn't make sense.

Only other thought....that light framed wood shearwall systems usually have many secondary shearwalls.

Also..their diaphragms may break up long before the seismic overstress would even develop in the collector.

Just a bunch of random thoughts - none of which seem to be a total answer....but I tried.

 

[Taro]

My understanding, based on reading code committee minutes, is that the overstrength design requirement for collector elements was largely a reaction to the 1994 Northridge earthquake. It is closely related to the redundancy/reliability provisions and was introduced in the 1997 NEHRP recommendations.

Historically building construction was very redundant, with many walls/frames dispersed throughout the structure. However, a trend developed in which engineers attempted to economize their designs by using only a few "superframes". Lack of redundancy, size effects, etc. were deemed to be a problem in 1994. If you have only one frame/wall on each side of a building and it fails, then ....

So penalties were put into the code to encourage designers to provide more redundant systems. The collector overstrength requirement is one of them. It makes it difficult to drag loads a long way, so it prompts the designer to add frames/walls. The light-frame construction succeeded in getting an exemption to this requirement based on acceptable past performance.

 

[JAE]

Taro - the redundancy point is a good one.

 

[whyun]

I lost my long post thanking both of you for your input. I like JAE's comment about design force on the collector limited by the diaphragm capacity. Analogous to the item 3 in the design of brace members.

I also liked Taro's comment about redundancy. Traditionally, wood framed structures had very good performance and redundancy that perhaps code writers did not want to extend the omega requirement to such smaller buildings.

I believe omega applies to flexible diaph with tilt-up walls or masonry walls as in warehouses, markets or small office buildings.

It is interesting to note, for wall out of plane anchorage design, strength level forces gets a 1.4 increase for designing steel elements and gets a 0.85 reduction for wood members. Why similar increase or decrease is not in the collector design is another topic all together.

Thank you JAE and Taro.