Instructions for proper use of rooftop equipment wind pressure calculator

HELP PAGE For the Engineering Express®

Rooftop Equipment Wind Pressure Calculator

Note: See Limitations & Conditions of Use at the end of this article.

 

Calculator Inputs

1) Ultimate Wind Velocity (Vult): This is the peak 3-second Ultimate gust wind speed. Consult your building department or engineer for governing codes. For more information on what wind speed to use for your project, click here or visit our Knowledgebase and search ‘wind speed’.

2) ASCE Exposure Category: A terrain classification based on the amount of wind obstacles and ground surface roughness due to natural topography, vegetation, and constructed facilities.

As a general rule:

Exposure Category ‘B’ applies to urban/suburban areas, wooded areas, or other terrain with numerous, closely-spaced obstructions (trees, buildings, etc.).

Exposure Category ‘C’ applies to open terrain with scattered obstructions, such as flat, open country or grasslands. If in doubt between Exposure Categories ‘B’ or ‘C’, should be used.

Exposure Category ‘D’ applies to flat, unobstructed areas and water surfaces. It is typically used for coastline regions, as well as for sites near smooth mud flats, salt flats, and unbroken ice.

For more information on exposure categories, Click Here or visit EngineeringExpress.com/Exposures

3) Mean Roof Height (MRH): This is the average roof height between the peak and eave of the roof. If roof is flat (0° slope), MRH is height from ground to roof level, or simply, the height of the building. For more information, click here or visit our knowledgebase and search ‘roof height’.

4) Input Unit Information: # of Units: This is the number of unique & separate (not on the same stand) rooftop units being installed at a particular site location. The calculator allows up to a maximum of 5 unique units be included as part of the “Order Certified Results” functionality. The calculator default is 1 unique unit. To change the # of units, simply select from the dropdown the number of unique units being installed at the particular site.

If multiple of the same unit are being installed at a particular site location, the unit information need only be inputted once so long as the clearance height is the same for each unit. If clearance height varies, you will need to input the unit information again with the different clearance height(s) to ensure the wind pressure calculations are accurate.

5) Name / Model #: Input the name or model number of the particular rooftop unit. This is a mandatory field, but does not affect wind pressure calculations. Name / model number is used to easily identify the unit during review by a building department official or third-party reviewer.

mechanical rooftop calculator dimension description
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6) Unit Depth: This is defined as the smallest horizontal unit dimension (i.e. the lesser of unit width and depth). In other words, the length of the shortest unit side. Note: Unit depth does not affect wind pressure calculations. Unit depth is only considered when determining the minimum required clearance height for your unit for sites located in Florida.

7) Unit Height: This is defined as the height from the bottom of the unit to top of the unit. Refer to manufacturer documentation for unit height.

8) Clearance Height: This is defined as the support structure clear height, or height from roof level to the bottom of the unit. This is typically the height provided by a curb or roof stand.

For sites in Florida, there is a minimum required clearance height based on unit depth (shorter dimension). The calculator will display the minimum required clearance height based on the unit depth you input. Exceptions exist to this requirement which may permit the Authority Having Jurisdiction (AHJ) to allow a lesser minimum required clearance height than what the calculator displays; because of these exceptions, we recommend checking with the AHJ for the final verdict on whether your support structure’s clearance height meets the minimum required clearance height. To allow for these exceptions, you may input any clearance height as an input.

*NOTE: For sites in Florida and outside of Broward and Dade Counties, and if your support structure is a curb, the minimum clearance is 8” per FBCB 1510.10. (In this scenario, ignore the calculator’s displayed minimum clearance). For all other Florida scenarios, follow the calculator’s suggested minimum clearance unless the AHJ permits an exception (check with the AHJ for the final verdict on allowable clearance height).

For more information on minimum required clearance height in Florida, see ecalc.io/clearances.

With all required input fields filled out, output wind pressures will be displayed. The option to “Order Certified Results” will also appear at the top and bottom of the calculator. Click either button to proceed to the order page. Calculator will report if any input fields are not valid. Follow calculator error message as directed to obtain wind pressures and have a professional engineer review and provide certified results for permit.

You will receive a signed and sealed hard copy or digitally signed copy of the results after they are reviewed by design professionals on staff.  Results are intended to be used as the structural portion which along with your other documentation to obtain a building permit which is to be inspected by others for final approval.

Fees are listed on the checkout page and may vary.  Discounts for frequent use are available through our tokens system.


Glossary

To see a full glossary of terminology used in this calculator and Help Guide, click here for our full glossary or search for the glossary in our knowledgebase.


Explanation of Calculations

Calculations performed by this calculator are as described on the following page:

Lateral Wind Pressure:                                                                         ASCE 7-16 References                        

Fh,ult = (qh)(GCr)(Af)                                                                                 (ASCE 7-16, 29.4-1, p. 322)

DPh,ult = Fh,ult / Af = (qh)(GCr)                                                                    (ASCE 7-16, 30.10, p. 380)

“qh = qz  using Kz at Mean Roof Height h”                                               (ASCE 7-16, 26.10.2, p. 268)

qz = 0.00256(Kz)(Kzt)(Kd)(Ke)(V2) psf                                                       (ASCE 7-16, 26.10-1, p. 268)

Kz is based on height z in ft and the equation/table specified right:         (ASCE 7-16, 26.10.2, p. 268 & Table 26.11-1, p. 269)

z is taken as height of unit centroid above roof level: z = h + (Clearance Height) + (Unit Height / 2)

Kzt is 1.0 for sites located on flat terrain                                                  (ASCE 7-16, 26.8.2, p. 267-268)

Kd is 0.85 for rooftop equipment                                                              (ASCE 7-16, Table 26.6-1, p. 266)

Ke is 1.0 for site elevations at sea-level. Applicable for all other sites.    (ASCE 7-16, Table 26.9-1, p. 268)

V = ultimate wind speed, Vult, in miles per hour                                      (ASCE 7-16, 26.5.1, p. 249)

GCr = 1.9 for lateral wind pressure (worst-case)                                     (ASCE 7-16, 29.4-1, p. 322)

DPh,ASD = ± DPh,ult  x 0.6                                                                           (ASCE 7-16, 2.4.1, p. 8)

“The [lateral] pressure shall be considered to act inward and outward.” (ASCE 7-16, 30.10, p. 380)

Uplift Wind Pressure:

Fv,ult = (qh)(GCr)(Af)                                                                                  (ASCE 7-16, 29.4-1, p. 322)

DPv,ult = Fv,ult / Af = (qh)(GCr)                                                                    (ASCE 7-16, 30.10, p. 380)

(Same qh as calculated above for lateral wind pressures)                      (ASCE 7-16, 29.4-1, p. 322)

GCr = 1.5 for uplift wind pressure (worst-case)                                      (ASCE 7-16, 29.4-1, p. 322)

DPv,ASD = ± DPv,ult  x 0.6                                                                           (ASCE 7-16, 2.4.1, p. 8)

“The [upward] pressure shall be considered to act in the upward direction.”   (ASCE 7-16, 30.10, p. 380)

 


Examples:

Example 1: Unit Mount to Roof Stand

Input Unit Information:

You are installing (3) of the same unit for this project, but they are each being mounted to a roof stand of the same height: # of Units = 1.

You input the model number for your unit. The unit width is 55.8 in. The unit height is 47.3 in.

The clearance height is the same as stand height: 34 in. (This satisfies Florida’s minimum required clearance height of 30in for a 55.8in wide unit).

Output Wind Pressures:

You should obtain ASD wind pressures of: ± 123.0 psf lateral and 97.1 uplift. The “Order Certified Results” button will appear if you would like to order these results to submit to a building department for permit.

Example 2: Unit Mount to Roof Curb

Let us assume we are installing the condition shown above: a rooftop unit mounting to a steel roof curb. The site is a 140 ft tall office building with a flat roof in Deerfield Beach, Florida. The site is a few miles inland from the coastline. The building has a road with other buildings on the south end, medium-sized parking lots (~100 cars per lot) on the east and west ends, and a grassy field on the north end.

The unit in question is a 28.7in tall, 35.5in wide, and 30in deep. The roof curb is 20in tall. What should we input to the calculator, and what should the calculator output?

Input Site-Specific Conditions:

Upon contacting the Broward County building department or engineer, you are advised to use 175 mph as your Vult.

The Exposure varies between categories B and C depending on the wind direction (if wind approaches from the south end, it would classify as Exposure B. If from the north end, it would classify as Exposure C. For the east and west ends, it could be justified as either Exposures B or C depending on the size of the parking lot and interpretation of the engineer/reviewing party). In order to ensure Exposure category is accepted without question, it is recommended to select Exposure Category C.

For a building with a flat roof, the mean roof height is equal to the building height: 140 feet.

Input Unit Information:

You are installing (2) of the same unit for this project, mounted to roof curbs of equal height: # of Units = 1.

You input the model number for the unit. The unit width is 35.5 in. The unit height is 28.7 in.

The clearance height is the same as stand height: 20 in. (This satisfies Florida’s minimum required clearance height of 18 in for a 35.5 in wide unit).

Output Wind Pressures:

You should see the ASD (Allowable Stress Design) wind pressures of: ± 103.7psf lateral and 81.9 uplift. The “Order Certified Results” button will appear if you would like to order these results to submit to a building department for permit.


Limitations & Conditions of Use

NO WORK SHALL BE CONSIDERED VALID FOR USE WITHOUT THE REVIEW AND CERTIFICATION BY A LICENSED DESIGN PROFESSIONAL.

THIS DOCUMENT & CALCULATOR ARE GUIDANCE ONLY FOR ESTIMATING, ASSISTING DESIGN PROFESSIONALS, & ESTABLISHING COMMUNICATION WITH CODE-COMPLIANT UNIFORMITY IN THIS FIELD.

» It is on the user of this calculator to ensure support structure height meets the required FBC minimum clearance height if the site is located in Florida. For more information, click here or visit ecalc.io/clearances.

» The “Rooftop Equipment Wind Pressure Calculator” produces allowable stress design (ASD) wind pressures based on ASCE 7-16, Section 29.4.1: Rooftop Structures and Equipment for Buildings, and is in accordance with the Florida Building Code Seventh Edition (2020) and International Building Codes 2015 & 2018. Results are also valid for ASCE 7-10, Florida Building Code Sixth Edition (2017), and International Building Code 2012.

» Design pressure results are in accordance with the codes and standards listed above and are applicable for rooftop equipment only.

» The following structures are not permissible for use with this calculator: Solid freestanding walls and signs, open signs, buildings, roofs, domes, chimneys, tanks, solar panels, and ground-mounted equipment. For these installations, please consult our other Design Aid Calculators or contact this office.

» Sample rooftop equipment that are permissible for use with this calculator include: mini-split units, packaged units, VRF units, generator enclosures, commercial chillers, and other split systems / mechanical equipment. Wall-mounted equipment mentioned prior that is mounted greater than 15 ft above grade are also permissible.

» Results are based on client-supplied data and are not field-verified by this office. Results are only valid with original or verifiable digital seal by a licensed Professional Engineer of this firm. Site-specific conditions and unit information shall be verified by others prior to permit and use. Selection of units’ respective product approval(s) shall be by others. Product approval(s) shall provide a performance rating capable of withstanding the ASD design pressures stated herein.

» ASD wind pressures produced by this calculator consider only the wind component of loading on the equipment. Other loading such as dead load is not considered. To consider full ASD load combinations as specified by ASCE 7-16 Section 2.4 and FBCB 2020 Section 1605.3, other loading considerations in addition to the calculated wind pressures will need to be considered.

Ultimate wind pressures may be obtained by dividing the allowable stress design (ASD) wind pressures given by this calculator by 0.6. Akin to above, full ultimate load combinations are not considered. Ultimate wind pressures derived by the stated method accounts only for the worst-case wind component of loading.

» Except as expressly provided herein, no additional certifications or affirmations are intended by use of this calculator or instructions contained herein.

» This calculator is for estimating purposes only and NOT for permit use. Results are only valid when reviewed, signed, and sealed by a Professional Engineer from Engineering Express. Click “Order Certified Results” in the filled-out calculator page to order a review and certification delivered to you for permit.

» In order for calculations to be valid:

  1. Mean Roof Height shall be between 15 ft (the minimum) & 700 ft.
  2. The building upon which the unit(s) are installed shall be situated on flat terrain. (Buildings in the immediate vicinity of hills, ridges, or escarpments are subject to different wind load calculations).

» *Florida only: Per FBCB Sections 1510.10 & 1522.3, roof-mounted equipment must be provided the minimum required clearance height based upon the unit’s width (i.e. longest side of unit) and whether or not the unit is located within/outside the High-Velocity Hurricane Zone. For more information, see: https://ecalc.io/clearances

» Please consider all above conditions before using this calculator.

Last Update: June 26, 2021  

April 5, 2021  Company  
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