Great article publised in US Glass by Sahely Mukerji quoting Stewart Jeske and some of our clients about important storefront issues. Read the short article here.
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Many blast project specs do not provide enough information to come up with the equivalent 3-second blast design pressure from ASTM F2248. Glazing contractors can save time by requesting the following information in advance.
- Explosive Weight – (Typically for most projects explosive weight II)
- Standoff Distance – (For most projects it is the Conventional Construction Stand Off = 82 ft)
- Level of Protection – (Typically Low Level of Protection)
The work JEI Structural Engineering did for the Hogle Zoo Acquatic project turned up some interesting research.
We hope you’ll enjoy these videos. It really highlights the importance of excellent structural engineering as we considered all aspects of glass impact. Find out more about JEI Structural Services.
The information provided on this page was published in e-glass weekly by a code consultant for the American Architectural Manufacturers Association www.aamanet.org on January 31, 2012. Read full article here. It’s an excellent summary of the code changes and recommended reading for all JEI clients.
2012 International Code requirements for windows and doors
A look at the International Building Code and International Energy Conservation Code stipulations for fenestration products. Courtesy of the American Architectural Manufacturers Association
The International Codes published by the International Code Council are the most widely used model codes in the history of U.S. construction codes. In addition to their widespread use throughout the 50 U.S. states, numerous federal agencies―including the National Park Service and the U.S. Department of Defense―have adopted the International Codes. Adoption of the ICC codes is also starting in other countries, particularly the Middle East.
The following is intended as a summary of the major requirements set forth for windows, doors and other fenestration products in the 2012 International Building Code and International Energy Conservation Code.
Manufacturers that sell products in several states should be aware that, at this point, multiple editions of the International Codes are being enforced by various U.S. jurisdictions. Most jurisdictions are currently using the 2009 editions of these two codes. As of fall 2011, the 2009 IBC is being enforced in 24 states, and the 2009 IECC is being enforced in 22 states. Local jurisdictions in other states are enforcing each of these as well.
There are, however, several states and local jurisdictions that are still using the 2003 or 2006 editions of these codes. Some states are even still enforcing the first edition of the International Building Code, from 2000.
It is anticipated that adoption and enforcement of the 2012 editions of the International Codes will begin within the next few months. Therefore, this article will focus on the 2012 requirements. Users of this summary need to be aware that other editions of the International Codes might be enforced by any particular jurisdiction, and therefore, this summary might not be consistent with the requirements of some jurisdictions.
The ICC website, www.iccsafe.org, offers updated information on which code edition is in effect in each state, as well as various cities and counties.
This summary is not a full discussion of the 2012 International Code requirements for fenestration products. It identifies relevant sections of the codes. For more specific information, obtain a copy of the relevant code or codes from ICC. It is also important to note that some jurisdictions adopt one or more of the International Codes, and then make their own amendments to them, at the state or local level. As a result, many jurisdictions have their own versions of these particular codes.
This summary does not attempt to address all of these variations of the base model code. In some cases, these jurisdiction-specific versions of the International Codes can also be obtained from the ICC. In other cases, the jurisdiction-specific versions must be obtained directly from that particular jurisdiction.
Recent changes
There are several significant changes between the 2009 edition and 2012 edition of the International Codes that relate to fenestration products. Cumulatively, these changes include:
- An update to the 2011 edition of AAMA/WDMA/CSA 101/I.S.2/A440-11 NAFS: North American Fenestration Standard/Specification for windows, doors, and skylights
- Removal of the distinction between metal and nonmetal framed fenestration in the prescriptive requirements of the IECC for commercial construction
- Addition of mandatory daylighting requirements, combined with automatic daylighting controls, in certain types of large, open public spaces in the IECC
- Updating to the strength design-based wind speed maps of ASCE 7-10 Minimum Design Loads for Buildings and Other Structures for the determination of design wind pressure in the IBC
- The addition of an exception to the minimum window sill height requirements for windows equipped with window-opening control devices in the IBC
- An increase in the minimum sill height required for operable windows in the IBC.
Testing and labeling of windows, doors and skylights
Exterior windows and doors are covered in Section 1710.5 of the 2012 IBC. This section requires windows and sliding doors to be tested and labeled in accordance with AAMA/WDMA/CSA 101/I.S.2/A440-11. The standard was developed jointly by the American Architectural Manufacturers Association, the Window & Door Manufacturers Association,and the Canadian Standards Association. The 2011 edition of the standard referenced in the 2012 IBC and IECC represents an update from the previous reference to AAMA/WDMA/CSA 101/I.S.2/A440-08 in the 2009 IBC and IECC.
As in the 2009 IBC, the latest edition of the standard applies to windows and “sliding doors” in the 2012 IBC. Other types of fenestration assemblies not included within the scope of AAMA/WDMA/CSA 101/I.S.2/A440-11, such as curtain wall and storefront, are addressed in Section 1710.5 of the 2012 IBC. These assemblies are to be tested to 1.5 times design load in accordance with ASTM E330-02, and the glass is to be designed in accordance with ASTM E1300-07e01.
Exterior swinging doors can be tested and labeled in accordance with AAMA/WDMA/CSA 101/I.S.2/A440-11 or tested to 1.5 times design load in accordance with ASTM E330-02. The 2012 IBC also permits garage doors to be tested to ANSI/DASMA 108-05, in lieu of ASTM E330.
AAMA/WDMA/CSA 101/I.S.2/A440-11 contains provisions for some types of exterior swinging doors. AAMA has put into place a program to certify these types of products for compliance with AAMA/WDMA/CSA 101/I.S.2/A440-11. This program depends upon testing of each proposed door assembly, rather than the component-based approach offered by ANSI A250.13 and others.
The 2012 IBC also requires unit skylights be tested and labeled in accordance with AAMA/WDMA/CSA 101/I.S.2/A440-11. Unit skylights are factory-manufactured fenestration assemblies intended to be installed in a single roof opening without intermediate framing members. Tubular Daylighting Devices are included within the definition of unit skylights in the 2012 IBC.
The requirements for skylights and sloped glazing occur in Section 2405 of the 2012 IBC. Section 2405.5 permits unit skylights to be evaluated for different positive and negative design pressures. This is unique to unit skylights. Skylights are subject to snow load as well as wind and dead load. The combination of these loads will often result in varying required ratings for positive and negative pressures on unit skylights.
The 2012 IBC requires exterior wall cladding systems―including curtain wall, storefront and punched openings―in high-wind areas to be subject to special inspections. The high-wind areas are determined by exposure category of the building. If the building is in Exposure Category B (surrounded by low- to mid-rise buildings), then special inspection is required if the design wind speed is 120 mph or greater. If the building is in Exposure Category C (open prairies) or D (near large bodies of water), then special inspection is required if the design wind speed is 110 mph or greater.
Special inspections, by definition in the IBC, are to be performed by people who are specifically qualified to inspect the installation in question. They are necessary only for the part of the system design that requires a registered design professional. So, for a curtain wall system, the special inspection would be of the structural components: the framing members, anchorage, joinery, etc.
Design loads
Provisions for design loads are set forth in Chapter 16 of the 2012 IBC. The design loads of concern for vertical glazing are design wind load and impact resistance. Skylights and sloped glazing are also subject to snow load and dead load.
Wind loads: The engineer of record for the project is to calculate the design wind pressure for components of the building envelope. The calculations are to be based on the design wind speed of the specific location where construction is to take place, the mean height of the building and its exposure. There are significant changes to the design wind load requirements for fenestration between the 2009 IBC and the 2012 edition due to changes to the wind load provision of ASCE 7 between the 2005 and 2010 editions.
The design wind load provisions of the 2005 and earlier editions of ASCE 7 were based upon allowable stress design of building components. The intent was to provide loads to which the building components had a fairly high likelihood of being exposed during the service life of the building. The building components were then designed to remain serviceable (i.e. not require replacement) when subjected to that load.
The 2010 edition of ASCE 7 provides design wind load provisions that are based upon strength design of building components. This method provides loads that have a lower likelihood of occurring during the service life of the building. The building components are then designed not to fail (rupture) when subjected to that load.
This change in methodology results in higher design wind speeds and pressures. At first glance, it might appear to require higher DP ratings for exterior windows, doors and skylights. In actuality, the 2012 IBC contains provisions to multiply this new, higher load by a factor of 0.6 for the purpose of conversion to the more traditional method of determining the design wind pressure based upon allowable stress design. It is very important that the builder, code official, manufacturer and anyone else involved in choosing or approving the windows, doors or skylights for a particular application understand that the higher design wind pressure provided by the 2012 IBC must be multiplied by this 0.6 conversion factor for the purposes of comparison to the Design Pressure rating of the fenestration product. In most, but not all, cases this conversion results in required design pressure ratings for fenestration that are roughly comparable to the more traditionally determined values. AAMA, WDMA, FMA and DASMA have published a technical bulletin on this topic that can be downloaded from the AAMA website at www.aamanet.org.
ASCE7-10 also provides three different design wind speed maps. The different maps are based upon the assigned risk category of the building being designed. There is one map for buildings whose collapse would present a low risk to human life, such as barns and storage facilities. There is a second map for buildings whose collapse is considered to be a moderate hazard to human life. Most buildings fall within this category. There is a third map for buildings whose collapse is considered a high threat to human life, and for those that are considered essential facilities. The former includes assembly or education buildings designed to house groups of 250 or more people, some medical care facilities and any other buildings designed to house 5,000 people or more. Essential facilities include hospitals, and police and fire stations, which are essential during emergency response situations.
The new maps result in higher design wind loads for buildings of moderate hazard to human life than for those of lower hazard. The highest design wind loads are for buildings whose collapse poses a high hazard to human life, and essential facilities. Previous editions of ASCE 7 and the IBC also required these types of buildings to be designed to higher design loads, but the actual increase was applied in a different manner.
Dead loads: The provisions for dead load in Section 1606 of the 2012 IBC are also based on ASCE 7-10. There are no significant changes to the dead load requirements for fenestration between the 2009 IBC and the 2012 edition of the same code.
Impact resistance: Section 1609.1.2 of the 2012 IBC outlines the locations where impact-resistant products are required. All exterior openings in wind-borne debris areas are required to be impact resistant in the 2012 IBC.
Determination of wind-borne debris areas in the 2012 IBC is similar to that of ASCE 7-10 and primarily defined by design wind speed. Since the 2012 IBC has three different design wind speed maps, some areas might be considered wind-borne debris areas for buildings such as essential facilities, but not for buildings whose collapse is considered to be of moderate threat to human life. In other words, in some parts of the country impact-resistant openings will be required for hospitals and police and fire stations, but not for office buildings and retail stores. It might be appropriate to provide a higher level of safety for the former buildings, but it will also require those selling fenestration products in those areas to be aware of this distinction and when it applies in their market.
Products that need to meet impact resistance requirements must be tested to one of a few different sets of standards.
One option is testing in accordance with ASTM E1886-05 and ASTM E1996-09, which must be used together. The 2012 IBC also permits the use of “other approved tests.” This may include Miami-Dade County test protocols, if approved by the authority having jurisdiction.
For residential applications, use of protective wood panels as an alternative to impact-resistant glazing or shutters continues to be permitted for limited applications. The 2012 IBC limits the use of protective wood panels to openings in one- and two-story single family dwellings, duplexes and residential care facilities.
JEI Structural Window Blast Expertise
Energy
Requirements for energy performance in both residential and commercial buildings are spelled out in the International Energy Conservation Code. Commercial buildings that comply with ASHRAE 90.1 are also considered to be in compliance with the IECC.
The 2012 IECC and ASHRAE 90.1-10 have similar formats. They both address the building envelope, mechanical systems of the building, lighting and hot water systems. Although the specific requirements for each of these systems differ between the two standards, they are considered to be very close with regards to the actual anticipated energy use of a commercial building built under either standard.
The 2012 IECC has two compliance paths for commercial construction. The first is the prescriptive path, which is the simplest to use, providing one set of energy efficiency requirements for each component of the building envelope.
The prescriptive path for commercial construction establishes maximum permitted U-factors and solar heat gain coefficients for fenestration. U-factor is to be determined in accordance with NFRC 100-04 or by use of a default table in the 2012 IECC. Similarly, the SHGC of the fenestration is to be determined in accordance with NFRC 200-04 or by use of a default table in the 2012 IECC.
Previous editions of the IECC contained separate U-factor provisions for metal and nonmetal framed windows and doors other than the main entrance door in commercial buildings. This distinction has been discontinued in the 2012 IECC. The 2012 IECC establishes maximum U-factors for fenestration based upon whether it is fixed, operable, or an entrance door. The figures below show the maximum U-factor and SHGC permitted for fenestration in commercial buildings.
Use of the prescriptive path in commercial buildings is limited to buildings where the vertical glazing and skylight area do not exceed certain limits. New to the 2012 IECC, these limits are dependent upon whether or not automatic daylighting controls are provided in the daylight areas of the building. Automatic daylighting controls reduce the artificial lighting load when daylighting is provided to a room or space. Combining automatic daylighting controls with well-placed fenestration allows fenestration to have a positive impact on the overall energy use of the building by reducing the lighting load during daylight hours.
If (1) a building is equipped with automatic daylighting controls, (2) at least 50 percent of the conditioned floor area is in a daylight zone, and (3) the glazing has a VT/SHGC ratio > 1.1, then 40 percent of the above-grade wall area of a commercial building is permitted to be fenestration area. If all three of these criteria are not met, then the fenestration area is limited to 30 percent of the above-grade wall area. Those parts of the exterior walls that are not included in the calculation of the window-to-wall ratio must meet the requirements of the 2012 IECC. For example, in order for those parts of a curtain wall system that are glazed with opaque glass to not be included in the calculation of the WWR of the building, they must be insulated as required for other metal-framed opaque walls in the building’s exterior envelope.
Similarly for skylights, if a building is equipped with automatic daylighting controls, up to 5 percent of the roof area is permitted to be skylights. If the building is not equipped with automatic daylighting controls, then skylights are not to exceed 3 percent of the roof area under the prescriptive provisions of the 2012 IECC.
Also new in the 2012 IECC are requirements for minimum skylight area. These provisions will require at least half the floor area of certain spaces to be toplit. These provisions apply to spaces that are directly under a roof, larger than 10,000 square feet in area, have ceilings in excess of 15 feet, and are used for offices, lobbies, atriums, concourses, corridors, storage areas, gymnasiums/exercise centers, convention centers, automotive service centers, manufacturing areas, nonrefrigerated warehouses, retail stores, distribution/sorting areas, transportation areas and workshops.
The 2012 IECC requires air leakage resistance of windows, door assemblies and unit skylights to be determined in accordance with AAMA/WDMA/CSA 101/I.S.2/A440-11 or NFRC 400-04, similar to the requirements in the 2009 IECC. The 2012 IECC also requires air-leakage resistance of curtain wall, storefront glazing and commercial doors to be determined in accordance with ASTM E 283-04.
Previous editions of the IECC used the same pass/fail criteria for air leakage of windows, door and unit skylights in commercial buildings as that established by AAMA/WDMA/CSA 101/I.S.2/A440. The 2012 IECC, however, establishes more stringent criteria than that currently in place for certain performance classes. Specifically, the maximum air leakage rate permitted for windows, sliding and swinging doors and unit skylights without condensation weepage openings is 0.2 cfm/sq. ft. when tested at 1.57 psf. The maximum air leakage rate permitted by AAMA/WDMA/CSA 101/I.S.2/A440 for some performance classes of fenestration is 0.3 cfm/sq. ft. when tested at the same pressure.
Emergency escape and rescue openings
The 2012 IBC requires emergency escape and rescue openings in sleeping rooms below the fourth floor of a building, and in all basements except those that are used only to house mechanical equipment and are less than 200 square feet in area. The 2012 IBC also contains some exceptions for rooms in buildings that are fully equipped with a fire sprinkler system, or for rooms that open directly to a corridor that leads to an exit in two directions.
Typically, the emergency escape and rescue opening requirements are met with operable windows or doors. Operable skylights and roof windows are also permitted to be used as emergency escape and rescue openings if they meet the size requirements and the bottom of their opening is within 44 inches of the floor below.
The requirements for sizes, locations, etc., are set forth in Section 1029 of the 2012 IBC. It is important to note that the required opening size of 24 inches high, 20 inches wide and 5.0 or 5.7 square feet in area must be met by “normal” operation of the window, door or skylight without the use of keys, tools or special knowledge, and without the removal of a second sash from the opening.
Minimum window-sill heights
The 2012 IBC requires the bottom of openings created by operable windows to be a minimum height above the adjacent interior floor when they are 72 inches or more above the grade outside the window. In the 2012 IBC, the required height of that window sill above the adjacent interior floor is 36 inches.
An exception is included, however, for windows that do not open more than 4 inches or that are equipped with window guards that comply with ASTM F2006-00 or ASTM F2090-08 or window opening control devices that comply with ASTM F2090-08. The window opening control device must limit the initial opening of the window to no more than 4 inches, but must also be releasable with no more than 15 pounds of force to open more fully. The intent of this later provision is to permit windows that are equipped with window opening control devices to also be used to meet the Emergency Escape and Rescue Opening requirements of the 2012 IBC.
Means of egress doors
Section 1008.1.7 of the 2012 IBC restricts the threshold height of the required exit door in residences and dwelling units to 1½ inches or ¾ inch, depending upon the type of door, from the top of the threshold to the floor or landing on each side of the door. The rise from floor or landing to the top of the threshold at other exterior doors that are not required to be accessible or which do not provide access to a Type A or Type B unit within the IBC, is limited to 7¾ inches, which is the riser height permitted for stairs.
Window installation
Section 1405.4 of the 2012 IBC requires window openings to be flashed “in such a manner as to prevent moisture from entering the wall or to redirect it to the exterior.”
Safety glazing
Section 2406.4 of the 2012 IBC establishes the locations where safety glazing is required. They include the following:
- Glazing in and near swinging and sliding doors
- Large lites of glass near walkways
- Glazing around tubs, showers, pools and similar fixtures
- Glazing near stairways, ramps and the landings for both.
In these applications, the glazing must be labeled per the Consumer Product Safety Commission CPSC 16 CFR 1201 requirements. There are some exceptions for applications that are considered less hazardous, such as very small openings in doors, decorative glass, and glazing provided with a protective bar, etc.
The previous exception for wired glass in fire-rated assemblies that complied with ANSI Z97.1 in other than educational-use groups has been removed. In the 2012 IBC, wired glass is only permitted in doors that meet CPSC 16 CFR 1201, just like any other type of glass. The 2012 IBC also permits the use of glass that meets the two most stringent categories of ANSI Z97.1 in hazardous locations that are defined within those codes, but which do not fall within the scope of the federal law established by CPSC 16 CFR 1201. These locations include tub and shower enclosures, door sidelites, large lites of glass, and glazing near stairs, ramps and pools.
The criteria for these two categories of ANSI Z97.1 are similar to CPSC 16 CFR 1201 for these applications, but ANSI Z97.1 was updated in 2004, while CPSC 16 CFR 1201 was last updated in 1977. Therefore, ANSI Z97.1 is considered to be more up-to-date and consistent with products currently available than CPSC 16 CFR 1201.
The defined hazardous locations did not change significantly between the 2009 International Codes and the 2012 IBC.
Replacement windows
As a general rule, when an addition is made to a building or a component within a building is replaced; the International Codes require the new component or addition to comply with the requirements of the current code for new construction. This is also true for replacement windows. The 2012 IECC requires replacement windows to comply with the energy conservation requirements for fenestration in new construction. This requirement applies whether the entire window unit―including frame, sash and glazing―is being replaced, or just the sash and glazing.
Sunroom additions
The 2012 IECC permits glazing in thermally isolated sunrooms to have a maximum U-factor of 0.45 in climate zones 4 to 8. By definition, a thermally isolated sunroom must be separated from the remainder of the building by either existing exterior wall construction or construction that meets the energy efficiency requirements of the 2012 IECC for exterior walls. The sunroom must also be equipped with a separate heating or cooling system or thermostatically controlled as a separate zone, if conditioned. Previous editions of the IECC placed size restrictions on thermally isolated sunrooms but these restrictions do not occur in the 2012 IECC.
Sunrooms must be thermally isolated from the remainder of the home to take advantage of the higher permitted U-factor for fenestration. Under the 2012 IECC, sunrooms can be built as part of new construction, but they must still be thermally isolated from the remainder of the home, as discussed above, to use the U-factor of 0.50 rather than 0.35 in climate zones 5 to 8, or 0.40 in climate zone 4.
Site-built glazing
Chapter 24 of the 2012 IBC references ASTM E1300-07e01 for glass design. The 2007e01 edition of ASTM E 1300 addresses several types of glass layups and support combinations that were not addressed in previous editions of the standard. Having it referenced in the 2012 IBC greatly enhances the designer’s options in terms of providing glazed openings that can meet all the requirements of the code, including energy efficiency and impact resistance.
A previous provision that requires glass to be designed by a registered design professional if the glass framing deflects more than L/175 or ¾ inch remains in the 2012 IBC. An exemption to this requirement continues to be given in Section 1710.5 of the 2012 IBC for exterior windows and doors that are tested and labeled in accordance with AAMA/ WDMA/CSA 101/I.S.2/A440-11.
Skylights and sloped glazing
The 2012 IBC has different requirements for factory-built unit skylights than for other types of glazed assemblies in roofs such as skylights and sloped glazing. Factory-built unit skylights that contain only one panel of glazing material are required to be tested and labeled for performance grade in accordance with AAMA/WDMA/CSA 101/I.S.2/A440-11 in the 2012 IBC. Section 2405.5 of the 2012 IBC establishes the required performance-grade rating based on the provisions of that code for wind, snow and dead loads.
As for vertical glass, glass in sloped glazing is to be designed in accordance with ASTM E 1300-07e01. The requirements for screening under skylights and sloped glazing, as set forth in Section 2405.3 of the 2012 IBC, are consistent with previous editions of the International Codes. This includes requiring the screening to be securely fastened to the framing and to be able to support twice the dead weight of the glass. Requirements for curbs on skylights and sloped glazing, when applicable, is also consistent with those in the previous editions of the International Codes, and are set forth in Section 2405.4 of the 2012 IBC.
Code cycles
As noted at the outset, this article focuses on the requirements of the 2012 editions of the International Codes. Heading into 2012, many states and local jurisdictions are still referencing older editions of the IBC and IECC. Adoption and enforcement of a new edition of a model construction code traditionally occurs most significantly in the second year after its publication. Therefore, we can expect 2012 to be a transition year, with some jurisdictions adopting and beginning enforcement of the 2012 International Codes, while others continue to enforce the 2009 or earlier edition of the International Codes.
Solar Roadways are being developed to pave roads with solar panels that you can drive on. Co-founder Scott Brusaw shows us an exclusive look at his most recent prototype.
Watch the video.
The work JEI Structural Engineering did for the Hogle Zoo Acquatic project turned up some interesting research.
We hope you’ll enjoy these videos. It really highlights the importance of excellent structural engineering as we considered all aspects of glass impact.
Polar Bear Attacks Glass at a zoo
Polar Bear attacks underwater viewing glass
For the past several years, the Unifed Facilities Criteria (UFC 4-010-01) has been the governing code for all U.S. Department of Defense (DoD) blast mitigation projects. Referencing the current ASTM F2248-03 standard, the UFC provides a guideline for determining an appropriate static design blast pressure for both framing and connections of blast resistant glazing systems.
Surprisingly, many engineers and glazing contractors are unaware of the requirements set forth by ASTM F2248-03 for the design of framing connections for blast resistant glazing systems. ASTM F2248-03 specifies connection design loads of at least 2.0 times the magnitude of the 3-second equivalent design load or the glazing resistance as determined from ASTM E1300, whichever is greater. Often the glazing system connections to the main structure are only designed to resist 2.0 times the 3-second equivalent design load, despite the glazing resistance of the system.
The UFC 4-010-01 is currently undergoing revisions that should clairify blast design loads and reference a more stringent version of the ASTM F2248 standard (ASTM F2248-09). The revised ASTM F2248-09 sets forth the following criteria for the design of blast resistant framing connections to the main structure:
a) 2.0 times the magnitude of the load resistance of the blast resistant glazing if the maximum air blast pressure is greater than one half the magnitude of the load resistance of the blast resistant glazing.
b) 1.0 times the magnitude of the load resistance of the blast resistant glazing if the maximum air blast pressure is less than one half the magnitude of the load resistance of the blast resistant glazing.
Currently, UFC 4-010-01 (2007 revision) references ASTM F2248-03 and not the more up-to-date F2248-09 edition. It is our understanding that ASTM F2248-09 is not required in the design of blast resistant systems until referenced in the most current version of the UFC which is anticipated this year or early next year.
The changes may be difficult to accommodate with static equivalent analysis and may require a larger push for dynamic blast analysis to maintain reasonable connections.
Written by Matt Quinlivan, E.I.T.
Matt Quinlivan, E.I.T.
Design wind load charts put out by manufactures are usually only good for estimating overall span deflection of a mullion, and do not typically consider proper methods for stress design.The 2010 Aluminum Code has new criteria for considering the unbraced length of open sections. The unbraced length for a vertical mullion is usually considered to be the distance between horizontal mullions. However, design wind load charts put out by many manufactures of storefront systems are often based on the assumption that the mullion has full lateral buckling support and an unbraced length of zero. How can this be?
I believe that the manufacturers are considering lateral bracing from the glass and the mechanical gaskets. However, after review of many industry specs it is clear that mechanical gaskets should not be considered as a means of lateral bracing for open shaped storefront mullions. Therefore, the charts error on the side of being too liberal. When it comes to the calculations, they can’t match up and the mullions usually don’t perform as well as the charts indicate.
Unfortunately, it’s in the interest of the manufactures to keep the charts the same because they are competing against one another for framing systems with the highest performance standards.
Glaziers should keep this in mind when selecting open shaped vertical mullions and stay well under the curve projections that are indicated. If glaziers use the charts, as is, reinforcing structural glazing or heavier mullions will likely be needed.
Authors:
Stewart Jeske, P.E.
Matt Quinlivan, E.I.T.
Find out more about Storefront Design from JEI Structural Engineering.
| LinkedIn Group Comments |
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Group: Building envelope designers
Discussion: Be Careful With Manufacture Storefront Design Charts
I totally agree with you. Experience shows that manufacturers of facade systems offer the product and show its characteristics, but do not take into account many factors, leaving a wide field of work for designers. Designing the mullion must take into account at least the following factors: It is a mistake to assume that the whole profile has the same characteristics. The mullion connection point is often weaker and should be checked separately. Posted by Façade Engineer in August, 2011 |
| LinkedIn Group Comments |
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Group: Aluminum Curtianwall, Storefront Extrusions
Discussion: Be Careful With Manufacture Storefront Design Charts
Normally, the local architectural office (hue approves the building) has to give you the official wind loud charts in the area. From my experience, as building envelope designer, I will recommend to take a 1.5 safety factor in your calculations for the mullion and traverse deflection. Posted by Technical Director at Vimetco Extrusion in August, 2011 |
| LinkedIn Group Comments |
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Group: Facade (curtain wall) engineering group
Discussion: Be Careful With Manufacture Storefront Design Charts
I couldn’t agree more. Specifically, these charts don’t take into account whether there are any intermediate horizontal that are essential to bracing of the vertical mullion to develop the spans as noted in those charts. Also some of the open channel mullions won’t be able to achieve the minimum allowable stresses used in those charts due to the local buckling issues. Posted by a Structural Engineer, P.E. in August, 2011 |
| All codes include factors of safety because of the uncertainty about the loading, materials, manufacturing and installation. I can see that you propose to take into account that the glass in the glazing pocket provides bracing for the mullion and as a result to increase the allowable stresses. However, i wonder if after many years of use, the gaskets that are holding the glass and creating a lateral support for the mullion will as effective as when testing a brand new window in the lab. Also, the most of the projects specs specify that any contribution of the glass to the bracing of a million shall be ignored. Further, as you noted when loading front set storefront with negative wind pressure, the glass is in the tension Zone and thus does not provide any bracing. The truth is that the most critical conditions are at the corners of the building with negative (suction) loads. As far as you point about using formulas with Ly/Ry vs the formula with torsional constant J. We are using the formula with J for tubular mullions as they give more realistic results. For the open channel sections, the J is so small that it wouldn’t have made any difference. Lastly, we are tasked with a structural design of a very thin walled aluminum sections to span ever increasing heights as the architects want to see more glass and less metal. The boundary conditions, especially at the head are less and less restrained for torsion with horizontals simply snapped in and not physically attached to the mullions, with compensation channels used to restrain a 12 ft tall mullion from fallout in weak axis bending of the comp channel. If the mullion buckles due to the lack of horizontals bracing it, a large glass pane may fall out creating a danger to the left of occupants. I’d say that I don’t see how the requirements of the code are “too conservative”. Posted by a Structural Engineer, P.E. in September, 2011 |
| The question poses a dilemma for us as a manufacturer: the user, be it glazing sub or architect, wants to know the size of the system given the span, mullion spacing, and wind load of a given situation. These charts allow that. And, as the original question posed, that by no means CANNOT answer all the questions.The horizontals for bracing are only one part of the answer, so is thermal expansion / contraction, so is transition to surrounding substrates, so is the glass being designed for the size / applicable wind load. There are other issues as well, seismic loads (when applicable), building movements, etc. All these can’t be charted, but have to be addressed individually and collated into a working solution with the intended system. When one specific system doesn’t qualify, then others means must be sought to resolve the issues on a given project’s set of conditions.
But for starters, it’s not a bad place to at least have a little info out there. The appropriateness of the final solution, is in the hands of the end user(s), be it glazing sub or architect who reviews / approves the shop drawings AND structural calculations. Once shouldn’t be done without the other, one would hope. Posted by Curtain Wall Manager at Technical Glass Products in September, 2011 |
| I agree with you that this is a good start to at least “lock in” the system. However, I think it should be noted (in fine print if necessary) that these charts are for mullions that are braced at the minimum spacing of “X” ft. If the horizontal spacing exceeds this value the use is to seek additional engineering assistance. Let’s face it, unless the specifications and/or design documents call for engineered storefront solution, these charts will be used as the basis of design and no further engineering will be provided.Posted by a Structural Engineer, P.E. in September, 2011 |
| Suggesting that wind load charts developed by the manufacturers list a maximum horizontal spacing is a great idea and is something that I feel should be done in order to eliminate any confusion of glazing contractors. However, this is an idea that may have difficulty catching on. If one manufacturer develops wind load charts indicating a maximum horizontal mullion spacing while all others do not, glazing contractors will be more inclined to go with the system where horizontal limitations are not shown. Basically, if manufacturers put limitations on their charts, their product could be viewed as inferior when compared to others.Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011 |
| Yes, I agree with you that it may be disadvantageous for a manufacturer to put this limitation on their charts. However, this is also a question of marketing vs. potential liability. If the contractor selects the mull size based on these charts and the mulls fail, the manufacturer may be found liable.Posted by a Structural Engineer, P.E. in September, 2011 |
| Limitations of manufacturer’s charts definitely need to be spelled out better. Right now manufacturer’s charts are very misleading and often get the glazing contractor into a position where they have ordered material and the engineering calcs will show that reinforcing and/or added horizontals are needed. And this is very frustrating for all involved. We usually get the comment that, “We’ve been doing it like this for years and have never had a problem.”It’s likely that many of the systems have never been subject to a design wind event and if it does, it’s many years down the road. And the failure that may be seen is probably in the seal as the mullions slightly buckle and there may be slight permanent deformation. At that point the user of the facility (not knowing who is responsible) may just live with it or have it replaced. This is hypothetical, but I have a feeling that its probably the way it goes.
What do you all think? I would also really appreciate comments from engineers who work for storefront manufactures. |
| I understand the concern about the liability. But anyone in their right minds who uses those charts and never consults a PE? That’s counter-intuitive, but you’re right, it probably does happen.So the next time our charts are updated, we’ll see if we can add a “take two aspirin and call a doctor… “, no, sorry, add a clause stating “use of these charts is should be done in conjunction with a review by PE for final approval of span, anchorage, and stress of members depicted in the charts. Sizing of members should not be considered FINAL until that review is complete.” That should work, right? I’d tell the architect they should check with their structural engineers or the manufacturers if there were any questions about any of this.
one thought here: our primary framing members are rectangles, so that’s a pretty efficient shape when it comes to stress, avoiding buckling issues. Given the limits of the glass manufacturers on the size lites they can produce, is the absense of bracing horizontals and thereby stress in the main mullion(s) really going to be a factor if using tubes or rectangles and limit the deflection to L/175 or 3/4″ / etc? I know on our custom shapes, the T’s and the I-beams scare the heck out of me for buckling, and we always get a PE review for them – in design / profile selection -just in case. But on the rectangles, I haven’t ever seen the stress be a factor. Is this stress issue more for alum or for steel (TGP primarily does steel framing, not alum). I think most owners would assume something bad’s happened especially if the glass were to evacuate the opening. First thought is, the material’s been damaged, and shouldn’t be reused. I think any glazing sub who walks up on a situation like that would take the prudent route and replace the framing, don’t you? Posted by Curtain Wall Manager at Technical Glass Products in September, 2011 |
| Many, many glazing contractors use the manufacturer’s charts and never consult an engineer. Very seldom are engineering calculations/seal a part of the requirements for submittal in the specifications for storefronts.The context of the discussion is really aimed at storefront sections that are not “closed” sections as identified in the aluminum code. They are the open section storefront shapes that have a clip in filler plate. When ever we follow the aluminum code requirements for calculations on these “open” shapes most of the time stress limitations control the design and not L/175 or 3/4″. I would agreee with you about the rectangular closed shapes – usully its the deflection limit that controls. But to be considered a “closed” shape it must be a monolithic extruded shape – it can’t have a snap in filler plate.
I think a statement like what you suggest would be good to have on the charts. In most cases for the “open” shape storefront mullions, the charts are very misleading. Contractors do end up in trouble over and over again when they use those charts for estimating/ordering and then need calculations to meet the submittal requirements of the specs. I’m not sure where you were going with your last question. In my previous comment, I certainly was not suggesting that a glazing contractor would or should reuse something that had suffered damage. I was pointing out that most of the storefronts that are constructed probably don’t see the wind load that they were designed for until many years down the road. And if there is any damage because mullions have not been appropriately designed by an engineer (accounting for the stress issue associated with open shapes), then the damage is probably very slight permanent deformation and the seals will probably stop working. At that point the owner probably forgets who is responsible for the poor performance of the storefront and just calls a glazing contractor to have it replaced. Posted by Stewart Jeske, P.E. JEI Structural Engineering in September, 2011 |
| I think 2 aspirins will do… albeit may not help a leaky windowJYou are right, the closed tubular members are pretty efficient in terms of lateral torsional buckling. My main concern with buckling is for open channel rectangular shapes that are used in screw spline storefront systems. However, there are case where long span curtain walls with 10 ft tall glass panes will also have considerable allowable stress reduction.
The horizontals spacing doesn’t have an effect on the deflection of mullions but on the allowable stress. In other words, the section may fail in lateral torsional buckling before it yields. That issue applies to all materials and has to do with the shape’s geometrical properties and lateral constraints such as horizontals. The concern is that not only that the member fails structurally but also that the system loses its water and air tightness and that the glass may fall out (which is a major concern). Any post buckled material should definitely removed and replaced. Posted by a Structural Engineer, P.E. in September, 2011 |
| I agree with you that this is a good start to at least “lock in” the system. However, I think it should be noted (in fine print if necessary) that these charts are for mullions that are braced at the minimum spacing of “X” ft. If the horizontal spacing exceeds this value the use is to seek additional engineering assistance. Let’s face it, unless the specifications and/or design documents call for engineered storefront solution, these charts will be used as the basis of design and no further engineering will be provided.Posted by a Structural Engineer, P.E. in September, 2011 |
| We’ve discussed this in our office many times. If manufactures would indicate a maximum unbraced length, confusion in the design process may be reduced. However, is this something that the manufacturers would actually do??? I would have to say no. As a result of the competiveness of the industry, putting limitations on the wind load charts would make their product seem inferior to other manufacturers whose wind load charts have a higher capacity due to the fact they assume a continuous laterally braced frame.Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011 |
| Storefront Manufacture Comments |
| These charts do not consider lateral buckling as was explained earlier and as noted in the notes in our Detail Manual. In reality, the glass lateral stiffness does provide some bracing when using our standard side blocks. There has always been the question of how much bracing does the glass provide.Some consultants will not allow the glass to provide any bracing and some will consider the glass to provide full bracing. Our testing has shown that there can be some lateral deflection but at these lower loads, you will probably not get permanent lateral buckling but it would be hard to convince some consultants unless we did a test of the exact unit in question. I would say if this consultant is going to be on the job we probably do not want to fight his recommendation. If there were no consultant, I would have no problem using this heavy weight mullion in this application.
Emailed by a Storefront Manufacture |
| I received the information above from one storefront manufacture regarding our issues with lateral buckling in open shaped storefront members. They basically say that their wind load charts do not take into account any lateral buckling due to the fact that the lateral stiffness of the glass provides some bracing for the vertical member. We do not necessarily agree with this in that, under loading and over time, it cannot be assumed that the mechanical gaskets holding the glass in place will not weaken. If these gaskets weaken over time, the glass can be free to move slightly and cannot be used for lateral bracing. This is one reason why project specifications often do not allow for mechanical gaskets and glass to provide lateral support.I want you to have this information so that you are not caught off guard in the future when using manufacturer’s open shaped systems (especially when we are dealing with large spans having few horizontal mullions).
Posted by Matthew Quinlivan, EIT JEI Structural Engineering in September, 2011 |
| I reviewed the module for lateral buckling using the shapes that you requested. None of the shapes work by the calculations. Additionally, the shear-block mullion does not work structurally either.Your choices would be to add horizontals which would decrease the un-braced length for lateral buckling or change to a tubular system that will meet the wind load requirements.
Simply stated, Aluminum manufactures would be shooting at a moving target if they tried to publish lateral buckling charts. The allowable stress of the mullion changes based on the length of the mullion and the dimension between horizontals. When projects require the need for need lateral buckling design, we priced into the project PE calculations and designed the project accordingly. Storefront Manufacture – Engineering Department |
| We do have a lateral buckling problem with our storefront shapes. The additional comments that I received are:The fact that since simple charts cannot be created to account for the lateral buckling equations, we add the note at the top indicating that the charts are based on full bracing support. It is possible that a continuous silicone seal along the glass would supply enough support and if the architect and consultant accept that solution we should not have a concern as long as the water path along the edge of the glass is not blocked into the sub-sill. The other option of additional horizontals would also work.
To answer the question on why the mullions without horizontals will go longer spans than the mullions with horizontals is due to the fact that the trapezoidal loading applies less load to the mullion when there are no horizontals. Since lateral buckling is not considered in the charts the mullion without horizontals will go higher. Storefront Manufacture – Engineering Department |
It’s important for glazing contractors to get information from the automatic sliding door manufacturers, in advance, on operational limitations for their systems.
Some automatic door manufacturers have stringent criteria on how much the supporting header and jambs can deflect from wind load and how much can be supported vertically on top of the header.
One automatic door manufacturer had a limitation of 3/8” maximum lateral movement where the header was supported by framing. Curtain wall framing members that supported the automatic sliding door system were checked for wind load deflection and found to have over 1 ¼” deflection. This required significant reinforcing at the header and the jamb to reduce deflection to within allowable limits.
Jamb deflections also need to be considered in the overall deflection of the system under wind load.
Finding out operational limitations of the system, in advance, provides a higher quality installation. Many glazing systems will not offer the stiffness required to adequately ensure operation of the automatic door system. Some automatic sliding door manufacturers’ information states that the header must be attached to “something rigid”. So the question becomes, “What do they consider rigid”? If door systems are installed and framing is deficient, it could void the manufacturer warranty and the glazing contractor may become liable. After investigating the definition of ridged with one particular manufacturer, it was suggested by their engineering department that the header could only move ½” laterally.
When glazing contractors communicate with automatic door manufacturers, it’s important to reach the engineering department and ask for the operational limitations for deflection of the supporting framing.
Clear upfront communication can save weeks of time, improve client satisfaction, and reduce the risk of liability down the road.
Review JEI Structural Engineering Storefront design calculation services
Authors:
Stewart Jeske, P.E
For interior glass railing one of the things you need to watch out for is glass railing on stairs that traverse floor levels. If the floors are designed to deflect with live load, care is needed to make sure load doesn’t transfer to the glass railing system.
We saw a project with glass railing that goes between floors and the top portion of the metal handrail continued to the second floor wall framing. There is the possibility that the second floor framing can deflect and if you have a continuous rail that goes between the second floor wall and is attached to the stair, load can be transferred into the glass causing breakage.
It’s more appropriate to break the railing at the floor transition or provide a joint that slides.