Steel Moment Frame Code Overview

Following is a brief overview of the governing code documents for seismic design of steel moment frames, intended to provide a brief, hierarchical, at-a-glance directory.

2006 International Building Code (IBC)

• Chapter 16—Structural Design
  
• Chapter 22—Steel
  

2005 American Society of Civil Engineers/Structural Engineers Institute (ASCE/SEI) 7: Minimum Design Loads for Buildings and Other Structures including Supplement Number 1

• Chapter 11—Seismic Design Criteria
  
• Chapter 12—Seismic Requirements for Building Structures
  

2005 American National Standards Institute/American Institute of Steel Construction (ANSI/AISC) 360: Specification for Structural Steel Buildings

2005 American National Standards Institute/American Institute of Steel Construction (ANSI/AISC) 341: Seismic Provisions for Structural Steel Buildings

• Section 9—Special Moment Frames (SMF)
  
• Section 10—Intermediate Moment Frames (IMF)
  
• Section 11—Ordinary Moment Frames (OMF)
  

2005 American National Standards Institute/American Institute of Steel Construction (ANSI/AISC) 358: Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications including Supplement Number 1

 

 

Haiti Earthquake: January 12, 2010

I've had a lot of non-engineer co-workers in the construction field as well as laypeople ask me about the Haiti earthquake and whether we could learn a lot about construction from studying the collapses and structural failures there.

First, a quick overview of the geophysical aspects of the earthquake: here's an informative link USGS Haiti EQ Jan. 12, 2010. It was shallow (at 8.1 miles) and in close proximity (15 miles) to the city of Port-au-Prince. This led to a lot of energy being released to the city's structures. The deeper the earthquake is (the longer the distance the energy has to travel through the earth's crust) the less energy reaches the surface. So the Haiti EQ released a significant amount of energy on the city of Port-au-Prince.

As far as the magnitude (M7.0) goes this EQ was not a huge one. The combination of the shallow nature, proximity to the city, poor construction and poor building codes was deadly. As far as the question about us being able to learn from the collapsed and damaged structures there, the answer is unfortunately that there is not a whole lot for us to learn there. We have learned most of those lessons in the past in our country. Surveying the structural damage in Haiti would be akin to time-traveling back and viewing the under- and un-reinforced structures that we have learned not to build anymore. In other words, the building technology, for various reasons (economic, bureaucratic, etc), was not state-of-the-art. In the past, we have built under- and un-reinforced structures in our country and have already learned most of those lessons.

Optimum Value Engineering (OVE)

Optimum Value Engineering (OVE) is a Next Generation, advanced framing technique for wood structures that results in lower material and labor costs and improved energy performance for the building envelope. Recent estimates have indicated that advanced framing techniques can reduce framing costs by as much as $1.20 per sq ft and reduce the amount of wood used for framing by 11% to 19%.

Some advantages of OVE are:

1. Less material is used. Substantial amounts of lumber can be removed from the wall and floor framing and standard module dimensions save cutting and waste on sheet materials. The result is lower costs, more efficient use of natural resources and reduces landfill waste.
   
2. Lower Labor Costs. Less material and fewer framing members translates to quicker construction and lower labor costs.
   
3. Lower heating and cooling costs. Fewer wall studs (thermal bridges) interrupt the insulation, thus making the insulation more effective and reducing heat loss. The wood stud has a greater conductivity to thermal flow than the insulation, and therefore provides an easy path for hot/cold to bridge the wall. Use of a single top plate also means the amount of wall insulation, relative to wood, is increased. Annual heating and cooling costs of a home can be decreased by as much as 30%.
   
4. More flexibility for future remodels and Tenant Improvements. With structural members more strategically and efficiently placed, there is less structure to work around.
   
5. Use of Engineered Wood Products (EWP's) decreases material usage, increase quality of construction by minimizing some of the variable characteristics of traditional sawn wood like cupping, bowing and edge wane and can be manufactured from fast-growing, underutilized, and less-costly wood resources.
   
6. No special tools are required.
   

   
    

Some disadvantages of OVE are:

1. More upfront planning is required.
   
2. There is a Learning Curve. Wall framing layout drawings may be required to guide inexperienced framing crews, but savings of hundreds or thousands of dollars are possible once crews are familiar with OVE techniques. Framers must pay attention to plans instead of simply using standard practices and quality control is more important. Crews may be slowed down until they are used to being careful to avoid using unnecessary studs, instead of just adding a stud
   
3. Need thicker decking, cladding and finish materials, design for acceptable deflections over longer span (7/16" floor sheathing typ) Floor joists may need to be deepened.
   
4. There is a perception problem whereas some view wider framing spacing as a sign of 'cheap' construction. This may true in some cases, but if the framing members are strategically placed where the load paths are, as in OVE, the structure can be stronger than a traditionally framed structure that uses more wood
   
5. Different size headers require cutting different size cripples over headers
   
6. May inspire more questions from building official
   

 

The techniques of OVE may be used independently or as a "whole structure" package; it is ok to cherry-pick. Some of the techniques of OVE are:

1. Wall studs, joists (framing members with wide flanges (I joists) help reduce the clear span of floor sheathing) and rafters spaced 24" o.c. maximizes framing material cost savings and aligned (stacked or in-line framing) from floor to floor: allows use of single top plate because top plate does not take any vertical load. Simpson strap may be used to maintain continuity.
   
2. All interior, non-bearing walls may be built with single top plate (in typical home saves equivalent to about 2 or 3 dozen studs) and no headers.
   
3. Right-sized headers: instead of sizing all headers for worst case load and span, more headers are sized for their actual load and span allowing smaller (e.g. multi-ply 2x headers). Headers may be insulated with rigid insulation or by applying batts on one side, thus insulating a typically uninsulated area of the building envelope
   
4. Overall, outside dimensions to 24" module cuts framing lumber by eliminating closely-spaced members to make up for a small, added dimension. In a 28'x40 two-story house the savings are equivalent to eliminating about 35 studs. If framing is stacked, substantially more savings can be realized.
   
5. Ladders at T-intersections (lay out blocking to avoid light switches and outlets). Cutting and nailing 3 pieces of blocking requires about same labor as installing 2 studs. Scrap pieces may be used (more than a stud is saved at each intersection). The joint is stiffened by horizontal blocking. Most important, insulator can continue insulation in the exterior wall past the partition framing in, forming a complete insulated blanket around the house and avoiding a hidden, uninsulated cavity.
   
6. Open corner framing: Drywall clips (subs may be unfamiliar with), may require more studs in high wind or seismic areas. The extra stud may be 2x4 in 2x6 wall or whatever is needed to receive the gyp board. With a 2-stud corner, 1 stud is eliminated. In all cases, the open cavity at the corner can be insulated along the wall, eliminating the need for the framer to insulate a closed cavity before the sheathing goes on.
   
7. Use some manufactured wood
   

Examples:

1. PATH Field Evaluation for for Habitat for Humanity in Anne Arundel County, Maryland
   
   1. (framing savings of approx $1.05 per sq ft were realized). Annual heating and cooling costs were reduced by 29.2%.
      
2. Centex Homes, Victoria, Minnesota: compared 2 2,000 sq ft homes, one traditional framing techniques, one OVE
   
   1. Installation and material costs $4,039 vs $1,003
      
   2. 68% to 75% more wall that could be insulated
      
   3. Heating and cooling costs $1,003 vs $710
      

OVE is a change from the status quo and does require working a little smarter. If designed and implemented properly it will maintain a high standard of structural integrity and quality, without sacrificing structural strength, while being built cheaper and with a wiser use of natural resources. In-line framing, stacking can be used by itself, at 16" oc for substantial savings. Pick and choose techniques that work for you. In order to be successful with OVE advanced framing techniques the contractor needs to be a 'thinker' rather than just an 'assembler'.