HomeMy WebLinkAboutB13-0164 REV1 Roof Letter.pdf MARTIN / MARTIN
CONSULTING ENGINEERS
August 30,2013
Mr. Ryan Magill
Vail Valley Medical Center
181 West Meadow Drive
Vail, Colorado 81657
Re: VVMC Multi-Specialty Clinic
US Bank Building Roof
13.0314.S.01
Dear Ryan,
Martin/Martin was asked to review the existing roof of the US Bank building on West Frontage
Road in Vail, Colorado for the addition of a new rooftop air handler to service the new Multi-
Specialty Clinic on the third floor of the building.
I performed a site investigation on Monday May 6, 2013 to field measure the existing roof framing
as no existing drawings were available. Based on my observations, the roof structure over the new
clinic consists of 28" deep open web joists spanning east-west approximately 44 feet between 18"
wide flange steel beams. The 28"joists were spaced between 2'-6" and 3'-0"on center. The joists
consist of 2L 2x2x3/16 top chords, 2L 1 3/4x1 3/4x3/16 bottom chords, and L1 %2xI '/2x3/16 web
members. A structural analysis of these bar joists indicated that the joists have a total load capacity
of approximately 80 pounds per square feet (psf). Subtracting out the weight of the roof leaves a
snow load capacity of 60 psf.
The manufacturer of the roof joists, Vulcraft, was also contacted to determine if existing drawings
were available. They were able to locate a bill of materials from the original construction in 1973
which showed that the joists in question are 28H8 joists spanning 43'-3 3/4". Using historical load
tables, we are able to determine that the snow load capacity of the joists is between 57 and 68 psf
depending on the joist spacing.
A design snow load of 60 psf is consistent with our experience with other circa 1970's building in
Vail.
The current Town of Vail snow load requirement is 100 psf for a flat roof Therefore,the US Bank
roof does not meet current design loads,but the International Building Code (IBC) does not require
existing structures to be upgraded to meet current codes unless certain conditions are met.
225 MAIN STREET, UNIT 0101 • EDWARDS, COLORADO 86132 • 970-926-6007
Vail Valley Medical Center—Multi-Specialty Clinic
US Bank Building Roof
August 30,2013
Page 2 of 2
The IBC (2012 IBC 3404.3) states that if a modification or alteration to the structure causes an
increase in load of less than 5%, then the existing structure does not need to be modified,provided
it is not deemed a dangerous condition by the building official.
The proposed rooftop air handler will be 1,281 lb plus a 150 lb power exhaust unit. The proposed
location as shown on Davis Partnership drawings dated August 30, 2013 show that an existing air
handler will be removed and the new air handler placed in nearly the same location. The unit will
be oriented with its length in the north-south direction,the unit will span over at least four(4)joists
and the increase in load on each joist will be 357 lb,which is less than 5%of the total load carrying
capacity of the joists.
Therefore, according to the IBC, it is acceptable to place the proposed air handler on the roof as
shown on the architectural drawings so long as the handler is oriented in the north-south direction
and the roof curb spans a minimum of four (4) roof joists. Some local reinforcement of the joist
chord will be needed to deal with local stresses in the existing joists and to support the roof deck at
the new openings.
Martin/Martin would like to caution VVMC that the although the installation of the new air handler
meets the requirements of the IBC, according to current practice, the roof is currently overstressed
by 50% and additional load from the proposed equipment will only exacerbate the weakness of the
roof structure. This is a significant existing overstress and is approaching a dangerous condition if
the roof experienced the 100psf snow load. Reinforcement options such as welding on bar or plate
to joists or strengthening options such as adding additional joists or beams, are available to
strengthen the existing structure to support current design loads as well as the new rooftop air
handlers. These options may be discussed at the discretion of the hospital management based on
budgeting and the anticipated remaining useful lifespan of the building.
Please call if you have any questions or would like to further discuss the options.
Sincerely,
Sean J. Molloy,P.E.
Senior Project Engineer