This has become a very controversial issue with much confusion over interpretation. There are numerous installations which are glass rail instalaltions without a rail on top of the glass. According to the ICC, this is not code compliant in a guard situation. However, as seen by the many existing applications, this is not being universally applied.
Here is the code reference in the IBC:
IBC 2407.1.2: Support. Each handrail or guard section shall be supported by a minimum of three glass balusters or shall be supported to remain in place should one baluster panel fail. Glass balusters shall not be installed without an attached handrail or guard.
This wording was put into place to protect the public in the event of an individual panel’s failure. In that situation, the remaining two glass balusters would remain in place to hold the railing and prevent people from falling.
This problem begins with the interpretation of glass balusters shall not be installed without an attached handrail or guard. Many installers — and inspectors — are taking this to mean that as long as a handrail is in place, the code has been met.
What is missing though is the limitation set out in the preceding paragraph:
IBC 2407.1.1 Loads. The panels and their support system shall be designed to withstand the loads specified in section 1607.7. A safety factor of 4 shall be used.
Cross referencing to section IBC 1607.7 regarding design loads we find this:
IBC1607.7.1 Handrails and guards. Handrail assemblies and guards shall be designed to resist a load of 50 pounds per linear foot (pound per foot) (0.73 kN/m) applied in any direction at the top and to transfer this load through the supports to the structure. . .
IBC1607.7.1.1 Concentrated Load. Handrail assemblies and guards shall be able to resist a single concentrated load of 200 pounds (0.89kN), applied in any direction at any point along the top, and have attachment devices and supporting structure to transfer this loading to appropriate structural elements of the building. . .
The IBC1607.7 clearly states that the load must be met by the top of the guard (42 inches). Glass balusters will not be able to meet the 800 (4 times 200) pound concentrated load without a cap rail.
Update: The IBC approved a code change for the 2009 publication that will eliminate the need for a top rail if laminated, tempered glass is used as the baluster.