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Re: str steel bldg cost estimates - brace frame vs. moment frame

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Based on some older jobs I did years ago, I would not find 20% to be out of the ordinary.  The usual factors that can drastically affect the cost difference are:

1. Aspect ratio of heigth to span; tall and skinny buildings are a much higher premium due to controlling drift; Basically you are spending dollars on stiffness for drift control.
2. Clear span of frame; interior columns versus full clear span seems to affect the price; if the interior columns are pinned at the top, the moment frames are generally heavier; most PEMB are pinned at the top of the interior columns
3.  Axial type bracing is still the cheapest form of bracing in terms of money to lateral deflection.  When you use moment frames, you are increasing member stiffness for drift control.  You spend a lot more money to keep a moment frame from drifting 1" than you do to keep a brace rod from stretching 1".
4.  Building with masonry and glass will have tighter limits and therefore, the 20% higher cost would be affected by tighter drift limits.

Hope this Helps
Ron Martin


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