DENVER AMENDMENT PROPOSAL 2015 INTERNATIONAL CODES

Transcription

1 DENVER AMENDMENT PROPOSAL 2015 INTERNATIONAL CODES UPDATED DEADLINE: JANUARY 9, 2015 NOTE: Each proposed Denver amendment to the International Codes must be justified. In order to be considered, the amendment proposal must address climate, clarity and/or cost. 1) Submitter Information Name: David Renn, PE Date: 1/8/2015 Rev. 6/16/2015 Organization (if Structural Engineers Association of Colorado (SEAC) applicable): Phone: E mail address: david.renn@denvergov.org 2) *Signature: * I hereby grant and assign to City and County of Denver all rights in copyright I may have in any authorship contributions I make to City and County of Denver in connection with this proposal. I understand that I will have no rights in any City and County of Denver publications that use such contributions in the form submitted by me or another similar form and certify that such contributions are not protected by the copyright of any other person or entity. Note: electronic signatures are acceptable. 3) Indicate which International Code (I Code) you propose to amend (please use acronym): IBC If your code proposal requires an amendment to another I Code, please indicate which other I Code and section(s) is impacted, and provide the necessary language within the proposal form. See section below for list of names and acronyms for the International Codes. 4) Be sure to format your proposal and include all information as indicated on pages 2 3 of this form. 5) Send your proposal to Community Planning and Development (CPD), attention: Jill Jennings Golich, via at jill.jenningsgolich@denvergov.org. An e mail submittal should include an electronic version saved with a file name as follows: 2015_public_amendment_proposal_Codname_SectionReference, where Codename_SectionReference is replaced using the code and section being proposed (for instance 2015_public_amendmnet_proposal_IBC_Section312 ). The only formatting necessary is BOLDING, STRIKEOUT AND UNDERLINING. Please do not provide additional formatting such as this will be done by CPD. Please use a separate form for each proposal submitted unless as allowed in 3) above. Note: All amendment proposals received will receive an acknowledgment, and will be posted on the Building Code website to allow the public review. Please check here if a separate graphic file is provided. X Graphic materials (graphs, maps, drawings, charts, photographs, etc.) must be submitted as separate electronic files in either PDF, JPEG or TIF format (300 DPI minimum resolution; 600 DPI or more preferred) even though they may also be embedded in your submittal. Acronym DBC AP IBC IECC IEBC IFC IFGC IMC Code Name Denver Building Code Administrative Provisions International Building Code International Energy Conservation Code International Existing Building Code International Fire Code International Fuel Gas Code International Mechanical Code Acronym IPC IRC NEC Code Name International Plumbing Code International Residential Code National Electrical Code

2 AMENDMENT PROPOSAL Please provide all of the following items in your amendment proposal. Your proposal may be entered on the following form, or you may attach a separate file. However, please read the instructions for each part of the amendment proposal. Code Sections/Tables/Figures Proposed for Revision: Note: If the proposal is for a new section, indicate (new) and (2011 Denver amendments), (previously in 2011 Denver amendments) and new Proposal: Show the proposal using strikeout or underline format. At the beginning of each section, include one of the following instruction lines: Revise as follows Add new text as follows Delete and substitute as follows Delete without substitution Revise Denver amendments to , and , and add , as follows: Section General is replaced in its entirety with the following: General. Design snow loads shall be determined using whichever of the following criteria produces the largest stress in the member(s) being designed. Design snow loads shall be determined in accordance with Chapter 7 of ASCE 7, as amended in Section of this code, but the design roof load shall not be less than that determined by Section In addition, design loads on uncovered parking decks shall include the requirements of Section of this code Criteria 1. Design snow loads, including drifting and unbalanced snow loads, shall be determined in accordance with Chapter 7 of ASCE 7, Minimum Design Loads for Buildings and Other Structures Criteria 2. Neglecting drifting, unbalanced loadings, and snow load importance factors, a uniformly applied roof snow load (p f or p s ) shall be: A uniformly applied load on the entire exposed area of 30 pounds per square foot for all structures in the Risk Categories III and IV of Table A uniformly applied load on the entire exposed area of 25 pounds per square foot for all other structures. Roofs used for parking of passenger vehicles shall be designed for a total non-reducible uniform load of 55 psf. This load need not be combined with other snow loads or other live loads applied to the roof. Section Ground snow loads is replaced in its entirety with the following: Ground snow loads. Ground snow load, p g, for use with the procedures of ASCE 7, shall be 2535 pounds per square foot. Section Snow load importance factor Modifications to ASCE 7 is added: Snow load importance factor Modifications to ASCE 7. The values for the snow load importance factor, I s, in Table of ASCE 7 shall be amended as follows: Risk Category III

3 Risk Category IV Section Snow load on uncovered parking decks is added: Snow load on uncovered parking decks. Uncovered decks used for parking of passenger vehicles shall be designed for a non-reducible uniformly applied design load of 55 psf. This load includes the combined effects of snow and live loads, and need not be combined with other snow loads or other live loads applied to the parking deck.

4 Supporting Information: The following items are required to be included. Purpose: The proponent shall clearly state the purpose of the proposed amendment to climate and/or clarify that are specific to the City and County of Denver (e.g., clarify the Code; revise outdated material; substitute new or revised material for physical, environmental and customary characteristics; add new requirements to the Code; delete current requirements, etc to reflect physical, environmental and customary characteristics that are specific to the City and County of Denver.) Revise snow loads to reflect the snow loads currently recommended by SEAC for the City and County of Denver. Reasons: The proponent shall justify changing the current Code provisions, stating why the proposal is necessary to reflect physical, environmental and customary characteristics that are specific to the City and County of Denver. Proposals that add or delete requirements shall be supported by a logical explanation which clearly shows why the current does not reflect physical, environmental and customary characteristics that are specific to the City and County of Denver and explains how such proposals will improve the Code. This proposed amendment is necessary to reflect environmental characteristics specific to the City and County of Denver. The 2011 Denver Amendments currently include recommendations from SEAC including the following: minimum uniform roof snow load of 30 psf for high risk buildings (Category III and IV) and 25 psf for other buildings; drifting and unbalanced snow load provisions in ASCE 7 using a ground snow load of 25 psf for all buildings, and increased importance factors for Category III and IV buildings when using ASCE 7 provisions. The SEAC Snow Load Committee is currently finalizing a 2015 report titled, which revises SEAC s 2007 report titled Colorado Ground Snow Loads. A ballot copy of this report was recently provided to SEAC s membership to review and to vote on endorsement of this report. At a SEAC general meeting on May 28, 2015, the SEAC membership unanimously voted to endorse this report. As a result of this vote, SEAC is proposing this amendment to revise Denver s snow load requirements to reflect the recommendations in this report, as described below. The previous design ground snow loads were based on a statistical analysis using a uniform hazard approach (i.e. the snow load that occurs on average every 50 years). However, new studies conducted by the SEAC Snow Load Committee have shown that using the 50 year load does not provide uniform protection against the risk of roof failure, meaning the probability of failure may be higher than expected. The new design snow loads are based on a uniform risk approach instead of a uniform hazard approach to provide the reliability levels targeted by the national standard ASCE 7. As a result of this new approach, the design ground snow is proposed to be increased to 35 psf for use in the ASCE 7 snow load provisions. The ASCE 7 procedures will then result in a uniform roof design snow load of 25 psf for most (typical) buildings (i.e. Risk Categories I and II), and a uniform roof design snow load of 30 psf for buildings in Risk Category III, which are the same as the current uniform snow load requirements for these same Risk Categories. The uniform roof design snow load for Risk Category IV will increase from 30 psf to 35 psf. Since the ASCE procedures result in essentially the same uniform snow loads as previously required by the Denver amendments, the uniform snow load cases are no longer required in the amendments. The new SEAC report recommends increased importance factors for Category III and IV buildings that are in line with the current Denver amendments, and this amendment proposal retains these increased factors. Substantiation: The proponent shall substantiate the proposed amendment based on technical information and substantiation. Substantiation provided which is reviewed and determined as not germane to the technical issues addressed in the proposed amendment shall be identified as such. The proponent shall be notified if the proposal is considered an incomplete proposal, and the proposal shall be held until the deficiencies are corrected. The burden of providing substantiating material lies with the proponent of the amendment proposal. A minimum of two copies of all substantiating information shall be submitted. Excerpts from the 2015 report are attached. Bibliography: The proponent shall submit a bibliography when substantiating material is associated with the amendment proposal. The proponent shall make the substantiating materials available for review., Structural Engineers Association of Colorado, 2015

5 Referenced Standards: List any new referenced standards that are proposed to be referenced in the code and provide a minimum of one electronic copy. Should the amendment proposal be recommended for inclusion in the amendment package, you must provide two hard copies. No new referenced standards proposed. Impact: Discuss the impact of the proposed amendment and answer the three questions below on the impact of the amendment proposal. The increase in ground snow load may result in a slight increase in the cost of construction due to the impact on the structural design. Note that this proposed revision does not significantly change the uniform roof snow requirements since ASCE 7 will result in the similar loads, except for Risk Category IV buildings. The increased ground snow load will change the design loads for drifting or unbalanced snow per ASCE 7. Effect of the proposed amendment on the cost of design: Increase Reduce No Effect Effect of the proposed amendment on the cost of construction: Increase Reduce No Effect Is the amendment proposal more or less restrictive than the I Codes? More Less Same Departmental Impact: To be filled out by CPD/DFD Note: The department shall indicate one of the following regarding the impact of the amendment proposal: Effect of the proposed amendment on the time to review: Increase Reduce No Effect Effect of the proposed amendment on the cost of enforcement/inspection: Increase Reduce No Effect

6 2015 Prepared by the Structural Engineers Association of Colorado (SEAC)

7 Structural Engineers Association of Colorado Prepared By The SEAC Snow Load Committee D. Jared DeBock James Harris Abbie Liel Jeannette Torrents, Chair May 2015 Copyright 2015 by the Structural Engineers Association of Colorado BALLOT COPY NOT APPROVED FOR FINAL USE

8 Executive Summary The Structural Engineers Association of Colorado (SEAC) Snow Load Committee has developed new ground snow loads to be used for structural design in Colorado. Philosophically, these loads represent a transition from a uniform hazard approach to a uniform risk approach for determining design snow loads. Previous loads, including those in the 2007 SEAC report, determine design ground snow loads based on a uniform hazard, i.e. the design value at a site is taken as the snow load that occurs, on average, once every 50 years (in other words, the mean recurrence interval is 50 years). However, due to the large variations in climate throughout Colorado, new studies conducted by this committee have shown that using the 50-year load as the basis for design does not provide uniform protection against the risk of roof failure. In particular, at plains sites, roof design based on the 50-year ground snow load may not achieve the reliability levels targeted by national (ASCE-7) standards, meaning that the probability of failure may be higher than expected. The converse is true in mountainous regions, where the reliability achieved when the 50-year load is used in design may actually be greater than required by ASCE-7. These patterns stem from differences in snowfall and accumulation throughout the state. In the mountains, snow builds up over the course of the winter producing large ground loads, but many of those sites exhibit fairly low variability in the loads from year to year. Snow loads in the plains tend to be dependent on large single-storm events, and they are highly variable from year to year. The computation of loads for a target risk of failure results in loads appropriate for strengthbased structural design. However, for convenience of use with ASCE 7, the loads presented in this report have been divided by the load factor (1.6) prescribed for snow loads in ASCE 7. Thus the loads are also appropriate for use with allowable stress design methods and for serviceability checks. Compared to the 2007 recommended loads, there have been small decreases in design loads in the mountains (approximately 10%), and increases in design loads in the plains (approximately 50%-100%). The increase in the design loads in the plains is consistent with historic and current practice, where design roof loads of 20 to 30 psf have typically been used. A number of other improvements have been made. First, the new analysis utilizes approximately seven additional years of weather data generated since the development of the 2007 map. In addition, improved relationships for snow depth and snow weight have been used. Third, a method of using all the data for large regions of the state to better predict rare events has been used. Finally, reliability targeted loads have been computed from available data at over 300 sites. These loads have been correlated with the altitude of the site and maps have been produced that provide parameters to compute the ground snow load given the altitude of the site. The mapping process introduces a geographic smoothing, or averaging, on top of the reliability-targeted loads, which smoothens some of noise inherent in the historical record. The recommended design loads are those that come out of the mapping process. This report provides detailed documentation of: (a) new mapped values, including comparison to previously used values, (b) the snow load and depth data used in the development of the maps, and (c) probabilistic approaches used to evaluate roof reliability and conduct spatial smoothing. The report also provides recommendations for use of these loads with ASCE 7 and for future improvements. BALLOT COPY NOT APPROVED FOR FINAL USE

9 Table of Contents EXECUTIVE SUMMARY... II TABLE OF CONTENTS... III PART DESIGN SNOW LOAD MAPS INTRODUCTION AND MOTIVATION The Need for a Revised Design Snow Load Recommendations and a New Map Objective of New Reliability-Based Design Snow Loads RECOMMENDED DESIGN SNOW LOADS FOR COLORADO Basis for Use Design Snow Loads Additional Recommendations FUTURE WORK PART 2. DEVELOPMENT OF 2015 DESIGN SNOW LOAD MAPS FOR COLORADO DATA ACQUISITION AND PROCESSING Data Sources Initial Data Processing METHODOLOGY TO COMPUTE RELIABILITY-TARGETED GROUND SNOW LOADS Test Structure Design Reliability Analysis Overview Uncertain Demand Variables Uncertain Capacity Variables Example Reliability Assessments MAPPING RELIABILITY-TARGETED GROUND SNOW LOADS Overview Importance of Correlations of Snow Loads with Altitude Correlations of Snow Loads with Altitude Methods for Mapping the Parameter K Expert Judgment Evaluation of the Proposed Snow Map COMPARISONS OF 2015 RECOMMENDED LOADS WITH PREVIOUS PROVISIONS Comparison of Reliability-Targeted Design Ground Snow Loads to 50-year Ground Snow Loads Comparison with Prior SEAC Recommendations WORKS CITED APPENDIX 1. DEVELOPMENT OF CONVERSIONS BETWEEN SNOW DEPTH AND SNOW WEIGHT A1.1. INTRODUCTION A1.2. COMPACTED SNOW AND SETTLED SNOW A1.3. COMPACTED SNOW SITES A1.4. SETTLED SNOW SITES A1.5. INTERMEDIATE SNOW SITES A1.6. TREATMENT OF UNCERTAINTY/VARIABILITY IN THE DEPTH-TO-WEIGHT CONVERSIONS A1.7. WORKS CITED APPENDIX 2. DEVELOPMENT OF GROUND TO ROOF WEIGHT CONVERSION FOR MONTE CARLO SIMULATION A2.1. INTRODUCTION A2.2. ROOF AND SNOW LOADS DATA SET A2.3. FITTING THE PROBABILISTIC MODEL A Step 1: Non-parametric Characterization of the Ground-to-Roof Snow Weight Conversion A Step 2: Fitting the Analytical Ground-to-Roof Snow Weight Conversion Model BALLOT COPY NOT APPROVED FOR FINAL USE

10 A2.4. DISCUSSION AND LIMITATIONS A2.5. CONCLUSIONS A2.6. WORKS CITED APPENDIX 3. DETERMINING PROBABILITY DISTRIBUTIONS FOR SNOW SITES A3.1. INTRODUCTION A3.2. SITE-SPECIFIC DISTRIBUTION FITTING A Background A Tail-Fitting Approach A Limitations of the Site-Specific Tail-Fitting Approach A3.3. DISTRIBUTION TAIL SHAPES INFORMED BY MULTIPLE SNOW SITES A Accounting for Systematic Biases in the Distributions that are Fitted to the Clustered Data Sets A Approach for Clustering Sites with Similar Distribution Shapes A3.4. DISCUSSION AND CONCLUSIONS A3.5. WORKS CITED APPENDIX 4. RECOMMENDATIONS FOR SERVICEABILITY LOADS AND IMPORTANCE FACTORS A4.1. INTRODUCTION A4.2. SERVICEABILITY LOADS A4.3. SNOW LOAD IMPORTANCE FACTORS APPENDIX 5. TABULATED DATA SUMMARY A5.1. INTRODUCTION A5.2. RAW DATA SUMMARY A5.3. ASSEMBLED SNOW SITE DATA SUMMARY APPENDIX 6. COMPARISON OF 2015 DESIGN GROUND SNOW LOADS TO PREVIOUS EDITIONS BALLOT COPY NOT APPROVED FOR FINAL USE

11 Part Design Snow Load Maps 1.1. Introduction and Motivation This report documents the basis for and motivation behind the 2015 SEAC design snow load recommendations, as well as the methods by which they are developed. The new snow load map is based on a detailed statistical analysis that aims to achieve uniform structural safety throughout the state of Colorado The Need for Revised Design Snow Load Recommendations and a New Map The design snow loads described in this report revise the existing maps for Colorado in a number of ways. First, new snow data are available since the last version of the recommendations and are now incorporated into the statistical analyses of Colorado snow loads. Second, the new map smoothens design values between nearby sites, ensuring that there are not unwarranted big jumps from community to community, and provides updated guidance on the determination of snow loads as a function of altitude that is based on the unique climate and conditions in our state. Finally, and most importantly, the new map revises design loads to ensure that the risk of roof failure, or alternatively, the structural safety and reliability are consistent across the state. This report quantifies safety in terms of the reliability index (or safety index), which is inversely related to the probability of failure; a higher safety index corresponds to lower probability of failure. ASCE-7 targets a safety index of 3.0 (0.13 % probability of failure in 50 years) for ordinary occupancy (Risk Category II) buildings under snow loads. The current SEAC 2007 design snow load recommendations, which are based on a uniform hazard of a 50-year mean recurrence interval (MRI) snow load produce reliability indices (or safety indices, ) ranging from approximately = 2 in the plains (2.3% probability of failure in 50 years) to = 3.5 in the mountains (0.02% probability of failure in 50 years) for ordinary occupancy buildings. The probability of failure is higher in the plains because there is larger variability in annual maximum snow loads in the plains as compared to the mountains, where snowfall and accumulation are more consistent from year to year. The large variability in annual maximum snow loads means that plains sites are more likely to experience loads that are significantly greater than their 50- year loads, causing roof failures. These discrepancies in safety and building performance across the state of Colorado are in need of resolution Objective of New Reliability-Based Design Snow Loads Safety is the primary objective of building codes (ASCE 2010). Therefore, the objective of the new snow load recommendations is to achieve uniform safety for snow loading across the state of Colorado. The target safety objective is a reliability index of =3 for, failure that is not sudden and does not lead to wide-spread progression of damage for Risk Category II buildings (ASCE 2010). This report develops loads that are calculated to ensure achievement of the target safety objective, an approach that is fundamentally different than current practice, which is to design for a uniform hazard (e.g. a 50-year snow load). As a result, the recommended design ground snow loads are decreased in the mountains and increased in the plains of Colorado to meet the uniform safety objective. 1

12 1.2. Recommended Design Snow Loads for Colorado Basis for Use The recommended design ground snow loads provided in this report are intended to be used with ASCE 7 procedures for structural design. As such, they provide design ground snow loads which must then be converted to roof snow loads according the ASCE 7 procedures, with appropriate treatment of roof slope, unbalanced snow loads and drifting. For convenience, the recommended loads are provided in map and tabular form. It is the responsibility of the engineer to check with the local authority having jurisdiction before using these recommendations Design Snow Loads Tabulated Design Values The recommended design ground snow loads are tabulated for 297 locations in the state of Colorado (Table 1.1). Table 1.1 provides the design ground snow load for the selected communities. The tabulated loads are appropriate for building sites at or near 1 the site where the load is tabulated and having equal or lesser altitude. Except on the eastern plains, for sites near a tabulated site, but higher in elevation, the design ground snow load should be computed by Equation 1.1. A site p g,site = p g,tabulated ( ) A tabulated 3 1.1, where pg,site and pg,tabulated are the design ground snow load at the building site and at the tabulated location, respectively, and Asite and Atabulated are the altitudes of the building site and the tabulated location in thousands of feet, respectively. The development of the design snow load values is described in detail in Part 2 of this report. On the eastern plains, use Equation 1.1 if the site in question is more than 500 feet higher than the tabulated site, otherwise no altitude based modification is needed on the eastern plains. Table 1.1 Tabulated design ground snow loads (pg) for the state of Colorado City/Town County Latitude (deg) Longitude (deg) Altitude (ft) Design Ground Snow Load (p g) (psf)* Aguilar Las Animas Air Force Academy El Paso Akron Washington Alamosa Alamosa Alma Park Amba Lincoln Near is a relative term, which is left to the authority having jurisdiction and/or the judgment of the engineer of record. As a rule of thumb, sites located on opposite sides of a mountain ridge, or with greater than five miles (in the mountains) of separation between them, or with greater than 1000 ft. of elevation difference between them should not be considered to be near to each other. BALLOT COPY NOT APPROVED FOR FINAL USE

13 City/Town County Latitude (deg) Longitude (deg) Altitude (ft) Design Ground Snow Load (p g) (psf)* Colorado Springs El Paso Columbine Valley Arapahoe Commerce City Adams Conifer Jefferson Copper Mountain Summit Cortez Montezuma Craig Moffat Crawford Delta Creede Mineral Crested Butte Gunnison Crestone Saguache Cripple Creek Teller Crook Logan Crowley Crowley Dacono Weld De Beque Mesa Deer Trail Arapahoe Del Norte Rio Grande Delta Delta Denver Denver Dillon Summit Dinosaur Moffat Dolores Montezuma Dove Creek Dolores Durango La Plata Eads Kiowa Eagle Eagle Eaton Weld Eckley Yuma Edgewater Jefferson Edwards Eagle Elbert Elbert Elizabeth Elbert Empire Clear Creek Englewood Arapahoe Erie Boulder Estes Park Larimer Evans Weld Evergreen Jefferson Fairplay Park Federal Heights Adams Firestone Weld Flagler Kit Carson Fleming Logan Florence Fremont Fort Collins Larimer BALLOT COPY NOT APPROVED FOR FINAL USE

14 Additional Recommendations Loads for Use in Serviceability Checks Roof snow loads that are determined from the design ground snow loads in this study are considered sufficient for checking serviceability limit states. Given the focus on safety (strength) in development of these loads, the loads may be more or less than would be ideal for deflection checks. Appendix 4 addresses serviceability loads in more detail. Structures Assigned to Higher Risk Categories ASCE 7 states that Risk Category III and IV buildings are intended to have safety indices of = 3.25 and = 3.5, respectively (ASCE 2010). However, the snow loading importance factors of Is = 1.1 and Is = 1.2 do not consistently meet these safety criteria for much of the state of Colorado, even when they are applied to reliability-targeted design snow loads. The ASCE 7 Is values are unconservative in the plains and slightly conservative in the mountains. Therefore, it is recommended that the snow loading importance factory (Is) be computed by Equation 1.4 for Risk Category IV buildings and be taken as the average of Equation 1.4 and the value 1.0 for Risk Category III buildings. I s = A , where A is the site altitude in thousands of feet. Development of Equation 1.4 is documented in Appendix 4. No change is recommended for the importance factor for Risk Category I buildings. Drifting on Plains and Other High Exposure Areas The committee recommends that engineers consider maximum roof drift widths greater than the maximum roof drift width specified in ASCE 7 for buildings with roof steps located in the plains or other sites with high exposure to wind and strong correlation between snow events and wind. Buildings with roof steps in those areas are known to experience roof drifts extending further than the maximum drift width that is specified in ASCE 7. This phenomenon occurs because large snow storms in those regions are often accompanied by long duration (on the order of days) winds oriented in a consistent direction. If the wind direction is perpendicular to the roof step, a large snow drift will form. Designing for snow drift widths beyond what is required in ASCE 7 is ultimately at the discretion of the authority having jurisdiction and the engineer. Sites Above Timberline This report does not recommend design ground snow loads for sites above timberline (or approximately 11,500 ft. of altitude 2 ). First, there are essentially no historical snow data records at above timberline locations. Second, the relation between ground and roof loads is likely very different due to the extreme wind exposure. Flat roof loads may well be smaller than recommended in ASCE 7, and drift profiles are likely more extreme (i.e., the length of the drift for a given height of drift will be larger than recommended in ASCE 7). Snow loads for 2 Timberline is defined as 11,500 ft. altitude in this study, although the precise altitude varies across the state. BALLOT COPY NOT APPROVED FOR FINAL USE