ASCE 7 Standards Committee

Proposal to Revise the 2005 Edition of ASCE 7

 

Submitted by:   Richard Davis, P.E.

FM Global

1151 Boston-Providence Tpke

Norwood, MA. 02062-9102

781-255-4780

Richard.davis@fmglobal.com

Please provide name, address, phone, e-mail

 

Submission date:  Original - 6/30/08 (Revised 9/16/08, 10/21/08, 1/20/09 and 2/27/09)

 

Considered by ASCE 7 Task Committee on: Wind_______

 

Task Committee Action on Proposal: _____________________

 

 


SCOPE:  ASCE 7-05 Revise Section 6.5.15.1

 


PROPOSAL FOR CHANGE: (please use strike-out and underline format – please also include related modification/proposed addition to Commentary)

 

Note:  Items highlighted in yellow are changes resulting from previous ballot

 

Add to 6.3 Symbols:

 

GCfr = product of external pressure coefficient and gust effect factor to be used in determination of wind loads for rooftop structures

 

Edit Section 6.5.15.1 as follows:

 

6.5.15.1 Rooftop Structures and Equipment for Buildings with h ≤ 60 ft (18.3 m). The lateral force Fh, on rooftop structures and equipment with A f less than (0.1Bh) located on buildings with a mean roof height, h ≤ 60 ft (18.3 m) shall be determined from Eq. 6-298. increased by a factor of 1.9. The factor shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of A f is increased from (0.1Bh) to (Bh).

 

Fh = qh (GCfr) Af (lb) (N)                  (6-29)

 

Where:

 

GCfr = 1.9 for rooftop structures and equipment with  Af  less than (0.1Bh). GCf shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of Af    is increased from (0.1Bh) to (Bh)

qh = velocity pressure evaluated at mean roof height of the building

Af = vertical projected area of the rooftop structure or equipment on a plane normal to the direction of wind, ft2 (m2)

 

 

The vertical uplift force, Fv,  on rooftop structures and equipment shall be determined as follows:

 

Fv = qh (GCfr) Ar    (lb) (N)                               (6-30)

 

Where:

 

GCfr = 1.5 for rooftop structures and equipment with Ar less than (0.1BL). GCfr shall be permitted to be reduced linearly from 1.5 to 1.0 as the value of Ar is increased from (0.1BL) to (BL).

Fv = vertical force on rooftop structure or equipment, in lb (N)

qh = velocity pressure evaluated at the mean roof height of the building

Ar = horizontal projected area of roof top structure or equipment, ft2 (m2)

 

The components and cladding pressure on each wall of the rooftop structure shall be equal to the lateral force divided by the respective wall surface area of the rooftop structure and shall be considered to act inward and outward. The components and cladding pressure on the roof shall be equal to the vertical uplift force divided by the horizontal projected area of the roof of the rooftop structure and shall be considered to act in the upward direction.

 

Move the following existing text from the Commentary Section “Figs. 6-8, 6-18, 6-19, 6-21 and 6-22” to a new section C6.5.15.1, edit and add as follows:

 

C.6.5.15.1

 

ASCE 7-05 requireds the use of Fig. 6-21 for the determination of the wind force on small structures and equipment located on a rooftop. Because of the small size of the structures in comparison to the building, it is expected that the wind force will be higher than predicted by Eq. 6-285 due to higher correlation of pressures across the structure surface, higher turbulence on the building roof, and accelerated wind speed on the roof. There is now a very A limited amount of research is available to provide better guidance for the increased force [Ref. C6-99 and Ref. C6-XXX]. Based on this research, the force of Eq. 6-285 should be increased by a factor of 1.9 for units with areas that are relatively small with respect to that of the buildings they are on. less than (0.1 Bh).Because GCfr the multiplier is expected to approach 1.0 as A f or Ar approaches that of the building (Bh or BL), a linear interpolation is included as a way to avoid a step function in load if the designer wants to treat other sizes. The research in Ref. C.6-99 only treated one value of A f (0.04 Bh). The research in Ref. C.6-XXX treated values of A f = 0.02 Bh and 0.03 Bh, and values of Ar = 0.0067 BL.

 In both cases, the research also showed high uplifts on the top of rooftop air conditioning units. although the net uplift on the units was not measured. The consensus of the committee is that uplift forces may be a significant fraction of the horizontal force. Hence uplift load should also be considered by the designer and is addressed in 6.5.15.1.

 

 

Add Reference:

 

Ref. C.6 - XXX Kopp, G. A., and Traczuk, G. (2008). “Wind Loads on a Roof-Mounted Cube,” BLWT-SS47-2007, Boundary Layer Wind Tunnel Laboratory, U. of Western Ontario, Canada.

 

 

 

REORG:

 


ASCE 7-10 Section 29.6 and Commentary New Section C29.6

 

Add to 26.3 Symbols:

 

GCfr = product of external pressure coefficient and gust effect factor to be used in determination of wind loads for rooftop structures

 

Edit Section 29.6 as follows:

 

29.6  Rooftop Structures and Equipment for Buildings with h ≤ 60 ft

The lateral force Fh, on rooftop structures and equipment with Af less than (0.1Bh) located on buildings with a mean roof height, h ≤ 60 ft (18.3 m) shall be determined from Equation 29.6-1 29.5-1, increased by a factor of 1.9. The factor shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of A f is increased from (0.1Bh) to (Bh).

 

Fh = qh (GCfr) Af (lb) (N)                  (29.6-1)

 

Where:

 

GCfr = 1.9 for rooftop structures and equipment with  Af  less than (0.1Bh). GCfr shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of Af    is increased from (0.1Bh) to (Bh)

qh = velocity pressure evaluated at mean roof height of the building

Af = vertical projected area of the rooftop structure or equipment on a plane normal to the direction of wind, ft2 (m2)

 

 

The vertical uplift force, Fv,  on rooftop structures and equipment shall be determined from Equation 29.6-2.

 

Fv = qh (GCfr) Ar      (lb) (N)                             (29.6-2)

 

Where:

 

GCfr = 1.5 for rooftop structures and equipment with Ar less than (0.1BL). GCfr shall be permitted to be reduced linearly from 1.5 to 1.0 as the value of Ar is increased from (0.1BL) to (BL).

Fv = vertical force on rooftop structure or equipment, in lb (N)

qh = velocity pressure evaluated at the mean roof height of the building

Ar = horizontal projected area of roof top structure or equipment, ft2 (m2)

 

The components and cladding pressure on each wall of the rooftop structure shall be equal to the lateral force divided by the respective wall surface area of the rooftop structure and shall be considered to act inward and outward. The components and cladding pressure on the roof shall be equal to the vertical uplift force divided by the horizontal projected area of the roof of the rooftop structure and shall be considered to act in the upward direction.

 

Move the following existing text from the Commentary Section “Figures29.5-1 and 29.5-2” (4th paragraph) to a new section C29.6, edit and add as follows:

 

C29.6

 

ASCE 7-05 requireds the use of Figure 29.5-1 for the determination of the wind force on small structures and equipment located on a rooftop. Because of the small size of the structures in comparison to the building, it is expected that the wind force will be higher than predicted by Equation 29.5-1 due to higher correlation of pressures across the structure surface, higher turbulence on the building roof, and accelerated wind speed on the roof. There is now a very A limited amount of research is available to provide better guidance for the increased force [Ref. C6-99 and Ref. C6-XXX]. Based on this research, the force of Equation 29.5-1 should be increased by a factor of 1.9 for units with areas that are relatively small with respect to that of the buildings they are on. less than (0.1 Bh).Because GCfr the multiplier is expected to approach 1.0 as A f or Ar approaches that of the building (Bh or BL), a linear interpolation is included as a way to avoid a step function in load if the designer wants to treat other sizes. The research in Ref. C.6-99 only treated one value of A f (0.04 Bh). The research in Ref. C.6-XXX treated values of A f = 0.02 Bh and 0.03 Bh, and values of Ar = 0.0067 BL.

 In both cases, the research also showed high uplifts on the top of rooftop air conditioning units. although the net uplift on the units was not measured. The consensus of the committee is that uplift forces may be a significant fraction of the horizontal force. Hence uplift load should also be considered by the designer and is addressed in Section 29.6.

 

 

Add Reference:

 

Ref. C.6 - XXX Kopp, G. A., and Traczuk, G. (2008). “Wind Loads on a Roof-Mounted Cube,” BLWT-SS47-2007, Boundary Layer Wind Tunnel Laboratory, U. of Western Ontario, Canada.

 

 

 

REASON FOR PROPOSAL: 

 

It is intuitive and already recommended in the Commentary that vertical uplift force also be considered. With the current language this is neither obvious, nor clear as to how that is quantified.

 Wind tunnel studies on rooftop structures were conducted in 2008 by the University of Western Ontario (UWO) for FM Global Research (proposed Ref. C.6-XXX). Tests were done at a total of 16 different roof locations, including the corner, perimeter and field of the roof. Parapets were not simulated. The dimensions of the roof top units modeled were 10 by 10 by 10 ft (3.05 by 3.05 by 3.05 m), with no space below the units.  The wall surface areas of the units were less than 10% of the lesser building wall area. The roof surface areas of the units were less than 10% of the building roof area. Measurements were taken with wind angles varied in 10º increments. That work shows that a significant vertical component of force exists. The test data shows that the maximum net vertical uplift force coefficient is 1.5 on the roof of rooftop units with roof surface areas less than 10% of the building plan area that they are on. Similarly, that test data shows that the maximum net horizontal force coefficient is 1.9 on the rooftop units with wall surface areas less than 10% of the wall area of the building that they are on.  Higher pressure coefficients for rooftop structures modeled were not limited to corner or perimeter areas of the building roof.

 Other supporting information may be found in existing Ref. C6-99 (ASCE 7-05). Wind loss experience relating to both the anchorage of, and the envelope for, rooftop structures and equipment has been significant. The design of the cladding for rooftop structures and equipment is not currently addressed. Since higher pressure coefficients for rooftop structures modeled were not limited to corner or perimeter areas of the building roof, using the wall and roof pressures required for that area of the building would not always be adequate for the cladding of rooftop structures and equipment.

 

WLSC Status:  The editorial changes contained in this ballot item are being presented concurrently to the ASCE 7 MC and the Wind Load Subcommittee (WLSC) due to the revised IBC schedule.  The WLSC deadline is due two weeks prior to the ASCE 7 Main ballot.  The results of the WLSC will be provided to ASCE 7 Main Committee members as soon as they are available.  Please note however that this revision has been vetted and approved by a number of the members of the WLSC with substantial expertise related to specifically to this item.

 

 

 

Proposals to revise ASCE/SEI 7-05 must be submitted using this form and are to be submitted electronically to Jim Rossberg, Secretary, ASCE/SEI 7 Standards Committee at jrossberg@asce.org