HomeMy WebLinkAboutB13-0511 Structural letter MARTIN / MARTIN
oeglaeff CONSULTING ENGINEERS
September 11, 2013
Revised October 16, 2013
Mr. Ryan Magill
Vail Valley Medical Center
181 West Meadow Drive
Vail, Colorado 81657
Re: VVMC ED Rooftop Condensing Unit
13.0445.S.01
Dear Ryan,
Martin/Martin was asked to review the existing roof of the 1979 Emergency Department building in
Vail, Colorado for the addition of a condensing unit to service the Emergency Department air
handler.
The existing drawings from 1979 indicate that the design roof snow load was 70 pounds per square
foot (psf). The current Town of Vail snow load requirement is 100 psf for a flat roof. Therefore,
the 1979 ED building 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. 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 to meet the existing building code,provided it is not deemed a dangerous condition by the
building official.
The proposed rooftop condensing unit will be 2,450 lb. The proposed location as shown on Davis
Partnership drawings dated September 10, 2013 show that the new unit will be located on the south
side of the 1979 roof adjacent to the expansion joint to the 2001 addition. The unit will be oriented
with its length in the east-west direction.
The new condensing unit will increase the load on the roof by 8% and although the strength of the
roof is adequate to support the new condenser and the original 70 psf snow load, since the increase
in load is greater than 5%, the roof needs to be checked for the current 100 psf snow load
requirement,which causes the roof to be overloaded by 25%.
If the unit is installed on an 18 ft minimum length curb as shown on the Davis Partnership
drawings, then the unit will span over at least 6 pan joists, thereby limiting the increase in load to
less than 5%and satisfying the IBC existing building requirements.
225 MAIN STREET, UNIT C1 01 • EDWARDS, COLDRADO 136 1 32 • 970-926-6007
Vail Valley Medical Center
ED Rooftop condensing unit
September 11,2013
Revised October 16, 2013
Page 2 of 2
Therefore, according to the IBC, it is acceptable to place the proposed condensing unit on the roof
as shown on the architectural drawings so long as the condenser is oriented in the east-west
direction and the roof curb spans a minimum of 18 feet or over six(6) concrete pan joists.
Martin/Martin would like to caution VVMC that the although the installation of the new
condensing unit meets the requirements of the IBC, according to current practice, the roof is
currently overstressed by 20% 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. Reinforcing
options such as adding steel plates to the existing concrete pan joists, 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.
If reinforcement options are not a consideration, then steps should be taken to ensure that the snow
load does not exceed the 70 psf design load. For the Vail area, the density of snow calculates as
approximately 30 psf. Therefore, the 70 psf design load equates to 28 inches of snow. VVMC
should mark a 28"vertical offset from the roof which can be used to measure the depth of snow on
the roof and the snow should be shoveled off the roof so that the snow depth does not exceed 28".
Please call if you have any questions or would like to further discuss the options.
Sincerely,
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Sean J. Molloy,P.E. <c,;
Senior Project Engineer