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Re: Architect cheating on structural calculations - where's the building depa...

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I've said this for years and I still believe it; the provisions for wood design was an afterthought that was intended by the SEAOC Seismology Committee (speaking only of the 97 UBC) to force wood into a "one method design's all" philosophy. The truth is that light framing (especially when considering residential construction)is a different animal. In Commercial/Industrial projects, interior walls are intended to be changed and moved around in order to satisfy those renting or leasing space. In residential construction, interior walls, while assumed to be permanent, change at the owner or future owner's discretion. This make homes dynamic, not necessarily their diaphragms. Imagine trying to remodel a home that has been designed using an envelope solution from the worst case of rigid and flexible; wind and seismic. Unless the full design package is protected (and very few are), the engineer who remodels the home will never be able to accurately restore the original load paths without removing the sheathing from every wall and calclating the shear based on the capaciy of the sheathing or capacity of the holddowns. When I performed contract plan check last year for a local firm, I had 30 flexible designs out of 30 submittals - no rigid analysis and we simply accepted an increase in base shear as suitable for compliance to the Alternative Base Shear analysis. It is not practical or cost effective to utilize a design method that provides so little advantage if any in the overall design that our efforts would be better placed working to improve the quality of construction and compliance with our work (load path and detail compliance). Simple things like using a 5-ply shear panel over a 4-ply panel can aleviate over-nailing problems. Using holddowns over straps provides better connections especially when exterior concrete holddown straps are used in areas where the soil is highly mineralized and exposed straps due to spalling of the concrete will rot out in less than five years leaving no uplift protection for shearwalls.

As far as the issue with cantilevered columns: I have always designed to deflection limits. I don't need to have someone tell me to increase the R value to 2.2 times the lateral force to design for stiffness. When this issue was questioned, the first response was that the Northridge Meadows Apartments failed at the embedded columns in the front entry to the garages until they learned that the columns were pinned and not fixed at the base. Professor SK Ghosh suggested we leave the R value at 2.2 to force engineers design to stiffness. At least SEAOC wrote a response later that they believed the increased shear should be added only to the line of shear where the columns exist rather than the entire structure in the direction the columns were designed to offer resistance.
Personally, I don't need Prof. Ghosh to get involved in my practice and penalize my clients because he fears that I might design the columns without error. I don't mean to be disrespectful of an engineer who has provided a great deal to our profession, but I am angered that his one comment in Northbrook Illinois (I grew up a few miles away) can have such a profound impact on our professional practices in California. The issue of lateral load is subjective. If we design wood shearwalls to .187Wd to 0.25Wd and consider the shearwall deflection or stiffness, then why must we design the columns for approximatly 0.46Wd (approximations)to obtain the same deflection - the owner pays the price of the additional steel AND the additional foundation necessary to resist the load.

One day the Structural community will wake up to discover that residential wood framed structures may behave similar to other materials, but require a very different means to design that provides for future flexibility when the next owner want to redesign. We can't build homes that can not be changed or remodeled. They won't sell and they don't offer the family within the possibility of expansion unless the design documents are maintained for the life of the house - even then the cost to remodel would be excessive and the gains minimal when you look at the performance of engineered home rather than prescriptively design ones.

Dennis S. Wish, PE

In a message dated 12/9/2005 1:19:59 PM Pacific Standard Time, Gautam_Manandhar(--nospam--at) writes:

I understand your fraustration about the rigid/flexible diaphragm because that goes against the general practice. However, the code is pretty specific about what flexible diaphragm is. Unless you can prove that the diaphragm is flexible by calc, I think the plan checker is correct in asking you to provide rigid diapphragm calcs. (Some of the cities now provide an alternative to rigid diaphragm analysis by using the simplified base shear.)

That is the preferred method I am using...... My gripe also includes simple one story steel frames, tube steel is preferred but not accepted in steel frames, but cantilevered tube columns are!! The stresses are so low in the steel frames because the deflection governs.

You indicated "My biggest gripe in general is that one or two story residences are seismically treated (plan checked) like major structures". My attitude has been that the earthquake does not know if you structure is major or minor. It will find the weakest link and create havoc if the link is weak enough. ( My understanding is that of the total damage in the Northridge earthquake, half the dollar amount was due to damage to single family homes.) Also from the point of view of the client, that single family home is his "Taj Mahal" and a major structure. Over 80% of the badly damaged structures I inspected after the Northridge quake (mostly residental and industrial) had shotty workmanship (missing nails, bolts , straps, interrupted chord members by the plumbers, missing and/or misplaced diaphragm to wall connections, etc.)

Nowadays, even when Structural Observation is not required and/or not specified on the "approved" drawings, some inspectors "request" it anyway, wrongly thinking they are off the hook.

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