Deflection Limits for Glass Rail & Glass Wind Screens
by Stewart Jeske, P.E.
Listen is as Stewart Jeske, PE and the JEI Structural Engineering team discuss Deflection Limits for Glass Rail & Glass Wind Screens.
Glass Rail / Wind Screen Problems:
During the initial design process for Glass Railing and Glass Wind Screens engineers struggle to find adequate deflection limits.
Glass Rail / Wind Screen Solution:
Document attached has a table that summarizes another engineer’s search for deflection limits. Our typical limits we employ for deflection of glass guards is h/50 or 1” under loads for guard design (50lb/ft or 200lb point load). But our standard does not really have a defined basis in code requirements.
Sometimes project specifications may spell out the criteria indicated, but most often nothing is mentioned for guard rails or wind screens. Be careful to check 08800 specifications. Sometimes they may list decorative glass or wind screens and glass railing and then they may be subject to deflection criteria indicated for typical 4-side support glass. Architects have pointed to that in past projects.
There is essentially no deflection criteria provided by code for deflection of glass wind screens (not guards) and decorative glass walls for wind loads. The deflection of these items will not be seen during a wind design event.
Listen in as Stewart Jeske, P.E. discusses deflection limits for glass rail & glass wind screens with the team of architectural engineers focused on glass & glazing system engineering.
A great discussion on LinkedIn followed this post. Here’s a summary:
Railing Manufacture Comments:
I viewed your You Tube segment with interest. As a railing manufacturer, we see the questions you raise every day. Wind loading for example is an issue when you enter markets such as Florida as the code requirements there for frameless glass railings are somewhat less forgiving than elsewhere. Meeting Florida Building Code for a single supported edge system is a challenge when you must factor not only against positive as well as negative wind loads but also large missile impact.
Glazing Contractor VP Comments:
I would like some solid information on code requirements for the use of laminated in lieu of monolithic glass in glass rails, guards and wind screens. Codes that I have found are open to interpretation both local and national. We get conflicting information from various sources. Can someone direct me a source I can rely on and refer my clients to?
Railing Manufacture Comments:
Glazing Contractor, you reference a continuing challenge the industry finds its self in every day of the week. What and which codes do I adhere to. Who has jurisdiction. Who do I go to for advice? What should I use to mitigate against the specification? These and many others are questions we are faced with continually. As a manufacturer, we always look towards the professional community for support in this matter although invariably the professionals are looking back in our direction for one and the same. If you have a specific project in mind I would be more than happy to offer some guidance or at least reference points you can point towards.
Flatt Glass Consultant Comments:
In the UK we have a code of practice BS6180 which sets deflection limits for glass or barriers. You can determine deflection limit by dividing the height by a number but there is also a maximum limit regardless of the dimensions. When calculating the deflection in glass for the various design loads the stress limit for the glass is rarely exceeded due to the deflection limit. The limit is based on how comfortable building users are observing the glass when it is loaded. The design loads are often more than the design wind load so the wind load is not generally a limiting factor. The loads are applied separately to the barrier because they are unlikely to occur simultaneously. If it is extremely windy who wants to stand on a balcony? The deflection and stress are calculated for common designs i.e fully framed full height barriers, infill barriers e.g between newall posts and free standing barriers i.e clamped at the bottom only. Other designs can be physically tested.
Railing Manufacture Comments:
Flatt Glass Consultant, having secured BS6180 approval for our Panel Grip aluminum balustrade system several years ago in the UK you are correct regarding deflection and the criteria surrounding it. At 25mm deflection under 0.36, 0.74 or 1.5Kn load at 1100mm any single edge system is reliant on the structural capability of the glass. Here in the USA however specifically in Florida although to be honest increasingly more and more down the eastern seaboard the requirement to perform against wind loading is becoming a factor. Florida Building Code requires the balustrade system to be tested to a positive as well as negative wind load over and above the normal applied load requirements expected and demanded by the ASTM standards. Additionally, the system has to mitigate against large and or small missile impact dependent on its location within the building. We are now seeing further interest in areas where tornadoes are a factor as the wind loading in these areas is potentially higher than those on the coast.
Glass Rail Design Engineer, Stewart Jeske, P.E.
Our engineers have been discussing this quite a bit: The IBC 2012 section 2407 covers requirements for glass in hand rails and guards, however deflection criteria are not specified in this section. Other tables in the IBC (Table 1604.3, footnote h) specify deflection limits for the 50 lb/ft distributed load and 200 lb point load (“human loads”) requirements on hand rails. There are no guidelines in the IBC for deflection of glass hand rail or windscreens wind load deflection (windscreens assume that they will only be loaded with wind pressure and not the 50/200 “guard loads”), the deflection limits of windscreens are dictated by the limitations of the IGU/laminate glass and its components (limit deflection to prevent the glass spacers and laminate interlayers to fail). In cases were the windscreens use monolithic safety glass, then the deflection limit becomes purely a serviceability concern (keeping the deflection below a value that will otherwise seem unsafe or uncomfortable by the building occupants) and is typically left to engineering judgement to accept or reject the deflections.
Other resources that cover hand rail/guard rail deflections are the ICC Evaluation Service’s Acceptance Criteria for Hand Rails and Guards, AC273, AC439 and ASTM E 985 standard.
In very few cases, the project specifications will have very specific deflection criteria for hand rails, wind screens and/or wind guards. In these cases, the project specifications could govern over the somewhat conflicting requirements available in the code literature for these nonstructural components.