Overdesign Primary Structure to save labor cost?

[jcali]

This pertains to low, multi-story building design (4 stories, steel frame with exterior masonry walls). Through a colleague, I heard that structural engineers in a large northeastern metropolitan area are designing very rigid primary structures in order to avoid as much "secondary" framing as possible because of labor costs. My understanding is that heavy rigid frames are used with very low deflection ratios to avoid the use of bracing. With these low deflection ratios come the recommendation that masonry be built tight to the structure. I guess the thinking is that clips angle braces at the tops of non-bearing walls are labor intensive. Also, no relief angles are used to support exterior brick walls at floor lines. This thinking seems very odd to me, to say the least. One of the issues, differential temperature movement, is not addressed by this approach. Have any of you heard of this or a variation of this approach?

-JCali.

 

[AggieYank]

that seems very odd not to have brace angles at the top of non load bearing CMU. Even with the low deflection ratios, when the CMU is tight to the structure, it is load bearing. I would NOT do that.

as far as using much heavier steel in place of braces every so often for lateral bracing, where I work, we use the braces. I've never heard of people using heavy framing instead of braces.

So I agree, it all seems odd to me.

 

[JStephen]

The idea that you'd optimize the steel design for lowest overall cost doesn't seem odd. In fact, it would seem odd if you didn't do that.

 

[AggieYank]

JStephen. Obviously lower cost is good. what I'M calling odd is that it costs less to have heavy framing with no braces as compared to light framing with braces. It's my understanding that saving on steel cost is better than saving on the labor involved with installing braces.

 

[jcali]

Lower cost isn't good if you're compromising the structure. The idea of building masonry tight to structural steel doesn't sound right, whether it costs less or not.

-JCali

 

[UcfSE]

It may in fact cost much less to have heavier framing members than to have light members with braces and more details and fabrication and erection costs. As an example, the AISC published an article a while back about the economical benefits of increasing columns sizes and saving the labor of using smaller columns that required stiffeners. The cost of heavier members in that case may offset the additional cost of stiffeners by a great deal. Along those same lines, using heavier framing members could conceivably save over the extra cost of more complex connections and additional labor. It just depends on the building. Typically in the US labor is very expensive compared to other countries where labor is cheap and materials are very expensive, so the rules of thumb for economical design are different as well. Now whether all this is a good design decision is up to the EOR.

 

[pappyirl]

"With these low deflection ratios come the recommendation that masonry be built tight to the structure."

Why is this recommended?

 

[jcali]

That's the part that puzzles me the most. I was hoping that others in the Eng-tips universe had heard of this and could tell me why.

-JCali

 

[JStephen]

Obviously, the design process has to create a suitable end product, or there isn't a point to it in the first place. But if economic analysis of some kind isn't present in the design, you have problems. I have dealt with engineers who just seemed to have no clue of the economic consequences of their decisions, and the owners paid through the nose as a result.

Ever wonder why in the old days, they made all those bridges and water towers out of latticed channels? Or why they stopped doing it?

 

[jcali]

Constructive input to the question is appreciated. Preaching the obvious is not necessary.

-JCali

 

[apsix]

Yes jcali, but what is obvious to one person may not be to another. I believe this should be a discussion forum for engineers with a range of level of experience.

 

[jcali]

apsix,

You're right. All constructive input is welcome. I'm sorry.

-JCali