Technical Manual for Design and Construction of Road Tunnels - Civil Elements
This is an updated version of FHWA Technical Manual for Design and Construction of Road Tunnels - Civil Elements.
This update is necessary to add section 16.10 SPECIAL CONSIDERATIONS FOR SUPPORTED CEILINGS/HANGERS that was unintentionally omitted in the previous version.
Numerous highway tunnels in the United States have suspended ceilings for ventilation purposes and in some cases aesthetics. These ceilings are generally supported by keyways in the tunnel walls and by hanger rods that are attached to the tunnel liner either by means of cast-in-place inserts or post-installed mechanical or adhesive (chemical) anchors. FHWA issued a Technical Advisory in 2008 strongly discouraging the use of adhesive anchors for permanent sustained tension or overhead applications (see Appendix I). Any use of adhesive anchors in road tunnels must conform to current FHWA directives and other applicable codes and regulations.
Examples of typical hangers and their components are shown in Figure 16.21. Some, such as the one that consists of threaded rods, turnbuckles and clevises, is designed to carry load only in tension, while one that is made from a structural tube is designed to carry both tension and compression.
The inspection of these hangers is important to tunnel safety and a rigorous and regular inspection program that considers importance and redundancy is strongly recommended to maintain an appropriate level of confidence in their long-term performance. Both types of hangers should be checked visually for any defect either in the hanger itself or in its connections.
One simple method used to check if the tension-only hangers are in tension is to strike each hanger it with a hammer and to listen to the resulting tone. A hanger that is not intension or one that has little tension will produce a dull, metallic thud when struck, while one that is in tension will have higher pitched tone. However, this method can only be considered a "pass-fail" check and cannot be used to determine if the hanger is carrying the required design tension.
Another method for checking if the hangers are in tension is to shake each hanger by hand. A hanger that is not in tension or has little tension will deflect significantly more than one that is in tension. This method can also be used to check if the tension-compression hanger is loose or not. However, the inspector should remember that a hanger that is very long can still deflect considerably due to its length, even though it is properly tensioned.
Hangers that exhibit a visible defect or lack of tension should be closely inspected and checked for structural suitability.
The repair of ceiling hangers depends on the particular type of defect. If the hanger rod, clevis, turnbuckle or connection pins are broken or damaged they can be simply replaced with similar components (Figure 16.22).
The repair of loose connections at the tunnel arch is of primary concern. The recommended repair for failed adhesive anchors is to replace them with undercut mechanical anchors typical examples of which are shown in Figure 16.23.