http://www.asce.org/Product.aspx?id=2147490021

Kudos to Dirk Kestner, who coordinated, cajoled, and compelled us (the many co-authors of the book) to all complete our assignments, and to all of the other individuals who made it happen. I co-authored the Steel section. There is a trove of information in the book. Hopefully it will find its way onto the shelves (and hands!) of every practicing structural engineer in the country and beyond.
 

As an optimist, I feel that maybe, just maybe, this initiative will lead to the rating system addressing the structure of a building in a meaningful way. In that spirit, I have prepared this – my attempt at pulling together the sustainable aspects of a building structure, and the structural engineer, into the vernacular of the rating system credits.

MR-x: Sustainable Building Structure

(DRAFT, by Jim D’Aloisio, November 2010)

1 Credit (or Prerequisite)

1. Engage the project’s Structural Engineer in the LEED planning charrettes, AND
2. Certify that the building’s structural elements and systems have been efficiently designed to minimize the material use while satisfying the structural and serviceability requirements for their intended purpose, AND
3. Coordinate the foundations and the building superstructure with meeting the requirements of the building envelope insulation and air barrier requirements, including providing details at foundations and slab edges to thermally isolate the interior slab from exterior exposures where appropriate, AND
4. Comply with one or more of the following, as they apply to the project:
   1. For portions of the building where concrete is used for more than five percent of the building structure: Use an average of at least 20 percent reduction of Portland cement in all cast-in-place concrete by use of supplementary cementitious materials, AND Address and minimize or eliminate thermal bridging for any and all concrete elements that extend into or pass across the insulated building envelope.
   2. For portions of the building where masonry is used for more than five percent of the building structure: Use an average of at least 20 percent reduction of Portland cement in all structural masonry units and cementitious grout by use of supplementary cementitious materials.
   3. For portions of the building where wood is used for more than five percent of the building structure: Design, detail, and construct the structural system using Advanced Engineering methodologies to minimize material use and jobsite waste production, and maximize cavity wall insulation opportunities.
   4. For portions of the building where hot-rolled steel is used for more than five percent of the building structure: Address and minimize or eliminate thermal steel bridging for any and all steel elements that extend into or pass across the insulated building envelope.
   5. For portions of the building where cold-formed steel framing is used for more than five percent of the building structure: Detail and construct the exterior building envelope systems to minimize thermal energy loss through the envelope.
   6. For portions of the building where a manufactured, panelized system (including SIPs and precast concrete sandwich panels) is used for more than five percent of the building structure: Coordinate the structural details to ensure insulation and air barrier continuity.
   7. For portions of the building where natural materials, such as straw bale, cob, or rammed earth is used for more than five percent of the building structure: Detail the building so as to minimize long-term degradation due to moisture and other environmental factors.
   8. For buildings with exterior brick, masonry, concrete, or stone cladding: Minimize the potential for thermal building energy loss through ties and anchors.

I’d be very interested in hearing any comments before I finalize this concept and send it to USGBC in late December. What do you think?

Structural engineers who design building structures should have a basic working knowledge of building science, and how a building’s structure influences heat transfer through the building envelope.

Radical or rational?

Common sense or controversial?

Or all four?

Please let me know what you think.


Structural Engineer’s Pledge to Improve Building Envelopes

• I will coordinate foundation and slab edge insulation schemes with project architects, and eliminate thermal bridges across the envelope insulation.
• I will design and detail perimeter structural conditions to minimize or eliminate thermal steel bridging across the envelope insulation, and coordinate with the envelope’s air barrier systems.
• I will review the architect’s details to identify any areas where steel penetrates the envelope insulation and any areas where the continuity of the air barrier is breached, and suggest modifications or alternate schemes.
• During construction, I will consider the execution of the envelope details to be as important as the structural and foundation details through communication, site visits, and, where appropriate, special  inspections.
• I will educate myself on buildable structural details that can improve the energy performance of building envelopes, and find opportunities to educate clients and contractors, as well.

I hereby take the pledge to do what I can, as a structural engineer, to improve building envelopes. Will you?

The FRP angle was fabricated with the steel by the structural steel fabricator. No problems reported. The GC and the CM both want me to come back in the wintertime after the building is completed, with my infrared thermal camera, so we can see that the detail is working. I’m quite pleased.

Below is an image of the spreadsheet. If you want a copy of the Excel or Quattro Pro file, send me an email – jad@khhpc.com. Feedback is most welcome.

First, the good. The structural engineer (from our office) used a detail in which the steel stud track hangs over the inside edge of the foundation wall, covering the top edge of the vertical foundation insulation, which is on the inside face of the foundation wall. These are not load-bearing studs. The sill anchorage is fine – a 5 1/2″ wide track still leaves plenty of width across the foundation wall for concrete screws. Take a look:

This will provide full insulation along the exterior slab edge. Way to go Dave!

The not-so-good involves the gap between the stud track and the top of the foundation wall. Not all the architects we work with understand the importance of creating a “sill seal” to maintain the air barrier continuity across this gap. On this project I specifically looked, and found it to be a GAPING void. Take a look:

Gap under stud track with no sill seal

Slipping paper in gap under stud track

If not addressed, this gap will result in significant air leakage for the life of the building. This is resolvable, but it’s much easier to seal the gap before the track is installed than after. This condition is the norm for probably 90% of buildings built in this country today. Call it a sad state of affairs – or a huge potential for energy savings.

For years, I’d thought that the continuous steel angles and plates that extend across a building’s wall and roof insulation didn’t transfer enough heat to worry about. After all, no one complained! And we had developed a set of details that seemed to work well – they were economical, allowed dimensional adjustment where needed, and – most importantly – provided good structural support. Details that provided thermal breaks were simply not part of our design repertoire. Unfortunately, I was very wrong in the assumption that those insulation-bridging plates and angles didn’t matter. They do.

Turn R-Values on their Heads
My mistake was in thinking only in terms of relative R–values, which is a measure of a material’s resistance to heat flow. Sure, I thought, the R-value of steel is very small, but even if it’s nearly zero, the algebraic sum of the areas of the materials times the R-values means that a small area of steel reduces the overall R-value of a wall by only a small amount. But this approach is incorrect. The way to determine the thermal loss of energy through an area of building envelope is to look at the inverse of the R-values – the U-values – which is a measure of a material’s thermal heat transfer. Comparing U-values, carbon steel conducts heat about 1200 times better than expanded polystyrene (EPS) rigid insulation. This means that if the EPS insulation in an exterior building wall is “bridged” by a steel plate across an area which comprises just one tenth of one percent of the wall’s total area, more heat can flow through the steel plate than the entire rest of the wall! In this example, the wall’s total effective R-value will be less than half of a similar wall with no steel bridging across the insulation.

Improving Building Energy Efficiency
From a sustainability perspective, structural steel has a lot going for it. Its high recycled content, its recyclability, and its ability to make durable, adaptable buildings make steel a good choice for building structures. But architects and owners are gradually becoming aware of the importance of the design and detailing of the thermal envelope on a building’s utility demand for heating and cooling throughout its service life. Structural engineers, who can frequently be sidelined during discussions about a project’s sustainability opportunities, need to play a major role in reducing thermal steel bridging.
The industry is getting feedback about building energy loss through thermal steel bridging in two ways:
• The use of infrared thermal cameras, which are becoming more common, reveal warm lines in the wintertime at bridging locations, including masonry lintels, relieving angles, and roof edge angles.
• Data from actual building energy use, as opposed to the results of energy models, which may or may not have modeled the thermal bridges appropriately. Utility bills don’t lie!

The actual energy performance of a wall or roof system depends a variety of things. Thermal mass can play a significant role, especially under conditions where there are wide swings in temperature from daytime to night. And of course, any discontinuities in the air barriers can allow so much building energy to escape as to render the R-values meaningless! But the main point should not be ignored: Steel that bridges across an insulated building envelope represents a significant loss of building energy, with a corresponding reduction of the assembly’s effective R-value.

It’s time to develop some new details. Perhaps we can modify existing details that have been developed to incorporate thermal separation that blocks the thermal steel bridge. Maybe in some cases a completely new detail will be necessary. In Europe, manufacturers have developed proprietary thermal bridge isolators – packages, similar to a pack of shim plates – that span across a thermal envelope, with rated capacities for shear, moment, and axial forces. Such products are not yet readily available in this country. Certainly, developing new details represents additional design time and possibly an increase in construction costs beyond the traditional installations. But there is an ongoing payoff in energy savings over time.

Finally, this issue begs the question: Who should be responsible for the calculation of the effect of thermal steel bridging on a building’s insulation? Well, if a project has a building envelope consultant, determining the building envelope configuration should be part of their work. For other projects, it’s not clear who the responsible party should be. Most structural engineers assume that the architect would provide them with feedback if they are doing anything that compromises the thermal envelope, but this is frequently not the case. This could explain, at least in part, why the problem of thermal steel bridging has gone on for so long without being addressed. I suggest that since this steel is detailed and specified by the structural engineer, that he or she understand the thermal implications, and work to minimize the bridging, or, at least, inform the architect or client about the magnitude. We certainly wouldn’t want a non-engineer revising the steel support details! It’s time for structural engineers to understand this problem, and develop solutions. That’s what engineering is all about.

R_EFF = A_TOTAL / (A1/R1 + A2/R2)

A1 and A2 are the areas of the steel and the insulation, respectively, which add up to A_TOTAL. R1 and R2 are the materials’ respective R-values.

Example:
Let’s consider a strip of exterior building wall 12 feet tall and one foot wide, with a quarter-inch thick steel plate or horizontal angle leg, extending completely across the insulation plane in the wall. Assume the insulation consists of rigid expanded polystyrene (EPS). The steel plate occupies approximately (0.25/12)/12 = 0.0017 or 0.17% of the wall area.

For the steel, A1 = 0.25 /12 = 0.021 sf, R1 = 0.0031 per inch

For the EPS, A2 = (12 – 0.021) = 11.98 sf, R2 = 3.7 per inch

R_EFF = 12 / [(0.021/0.0031) + (11.98/3.7)] = 1.2 per inch

So, the effect of a 1/4-inch thick continuous steel plate, 12 feet on center, reduces the effective R-value of the EPS from 3.7 per inch to 1.2 per inch – a decrease of 68 percent.

This is the equation – this is the math – this is the basic concept that architects and structural engineers need to understand if they have any expectation of designing energy efficient buildings.

While it’s great to see such work, and validating to know that I’m not barking down an empty alley, it’s sobering to realize that the U.S. is in such a different place on this. We have, to my knowledge, no such specific information or codified requirements for limiting thermal bridging. The energy codes that I’ve seen define minimum R-values for the building elements, with the implicit assumption that the designers understand how to account for the effect of thermal bridges. Big assumption!