Sunday, August 22, 2010

Light Gauge Metal Stud Bearing Walls in Type II FR Construction - Benefits and Challenges

Over the last few years, we have seen a significant increase of the construction of new buildings constructed with light gauge metal stud bearing walls.  Typically this form of construction has been limited to single family homes and low rise condominiums & apartment buildings (i.e. 3 to 4 story).  Note that this type of construction is typically limited to Residential Occupancies, where long spans do not exist and the interior walls are unlikely to move (as in an office building).

Originally these building employed plywood shear wall sheathing and plywood floor sheathing (i.e. Modified Type 3 Construction).  In 3 or 4 story condominiums, steel braced and moment frames have been more recently been used, in a variety of forms, to provide the lateral force resistance for the building, but still with plywood floor and roof diaphragms.

Reilly & Company has taken this technology to it's next level of Innovation, 5 stories of light gauge metal stud bearing walls in a 6 story Type II FR building (i.e. concrete over metal deck floors instead of plywood and totally non-combustible).  Compared to the Concrete and Conventional Steel normal options, this provided the Sustainable - Efficient - Cost Effective solution.

The advantages of this type of construction are as follows:
1.    The Building is typically lighter than a wood framed building;
2.    The metal studs stand straighter than wood studs;The metal studs install faster than wood studs and joists;
3.    Metal stud framers (typically dry wall installers) cost less than wood framers;
4.    The building is more fire resistant and less prone to mildew (or dryrot) as compared to wood framed buildings;
5.    With the use of open web light gauge metal joists, the installation of plumbing, electrical wiring and mechanical ducting is easy; and
6.    If compared to concrete or conventional steel frame construction (Type I or II),  light gauge metal studs with concrete/metal deck construction can be approx. 30% less in cost for the structure.

The dis-advantages of this type of construction are as follows:
A.    Attention to detail is paramount in the proper construction of light gauge metal stud bearing wall construction, because of the inherent weaknesses of the material:
a.   The open web light gauge metal joists need to be aligned so that the preset holes in the webs align;
b.   All floor joists must be placed directly above each bearing stud, since the top track has essentially no strength to support the weight of the joists;
c.   In addition to the blocking between the joists above the studs, it is advisable to install a vertical section of stud (squash block) above the top track screwed to the side of each joist, due to the potential of the joists to buckle at the stud support;
d.   The studs must firmly bear on the curved bend of the top and bottom tracks (i.e. the studs must be completed seated into the top and bottom tracks) so that the screws connecting the studs to the tracks do not fail under the vertical load and so the flanges of the tracks do not buckle under the load; and
e.   Plumbing, conduits or mechanical ducting should not be permitted within any stud bay (let alone cutting through a stud or top or bottom track).  This means that all plumbing, conduits and mechanical ducting should be installed in shafts located outside of the bearing walls.

B.   Unlike wood, Metal is highly conductive of heat and cold.  As such,  light gauge metal studs require a much higher level of insulation on the exterior walls, including the requirement for rigid insulation on the exterior of the wall.  In addition, due to this high conductivity, it is advised, even in mild climates to install a vapor barrier (in addition to the waterproofing on the exterior side) on the warm side of the exterior studs to prevent condensation from occurring in the stud bay.

C.    Sustainability – in Modified Type 3 construction is not a reality.  Steel has a significantly higher negative impact on our environment than either concrete or wood, due to the high level of energy needed in fabrication.  Even though steel can be recycled, recycling is not without significant environmental cost.  Energy use from Deconstruction, Transportation and Re-forging is not insignificant.  Add to that the fact that steel cannot be re-forged back into steel without the addition of virgin steel (around 7% to 10%).  At least, recycled steel does not end up in a land fill.  However, if compared to conventional steel frame construction, light gauge metal stud bearing wall construction is certainly the Sustainable solution due to the partitions serving also at the supporting steel frame.

D.   Sound Transmission:  In an Apartment project of this type that we designed, the client wanted the concrete floors to be exposed stained concrete.  This added to the challenge to conform to the IIC50 and STC50 sound transmission requirements.  Partially since this type of construction is new there are presently no tested fire assemblies for these ratings. After an International search, scouring the world for sound assemblies, we found a sound assembly that would comply with these requirements with field testing.  Just in case we had problems with compliance, we were prepared to include wood flooring over insulation.

With this background, we were the first Engineering Firm to design a 6 story building with 5 stories of  light gauge metal stud bearing wall in a Type II structure (i.e. concrete/metal deck floors).  In a tight urban setting, being willing to be Innovative was crucial.  Our options were conventional concrete (which is no longer an economic solution), post-tensioned concrete (not a viable solution within our tight urban setting, surrounded by buildings), conventional steel frame construction (which, along with the concrete solutions, was too expensive), or try something completely different.

Considering that in an apartment or condominium, the light gauge bearing walls could easily serve double duty as bearing walls as well as partition walls and resulted in a significant savings in the cost of the structure.  The gauge of the studs ranged from 18 gauge at the top floor to 12 gauge at the lowest floor.  If you are in a high seismic zone (i.e. California), you will find that the economical solution is to use steel braced or moment frames to resist the high seismic forces.

Reilly & Company is proud of the Innovative Designs and Procedures that we have advanced over the years that produce the most Efficient/Sustainable designs that save construction cost.  Dennis Reilly was recently commissioned to present his Innovative work on Sustainable Design of Post-tensioned Structures at the Post-tensioning Institute (PTI) National Conference.

Sustainable Design should not cost more to build.  Viewing a building Holistically and Organically is the key to designing Sustainable, more Efficient, Lower Cost buildings.