Anchor Rods for Thermal Growth of Horizontal Vessels
Anchor Rods for Thermal Growth of Horizontal Vessels
(OP)
My company does a lot of foundation designs for horizontal vessels, and I am looking for some insight on anchor rod design for these vessels. The vessels typically sit on two saddles, one on each end. One of the saddles is slotted to allow for thermal growth of the vessel. We normally put a slide plate under the sliding saddle, and use grout under the fixed saddle. We design the foundations for a thermal growth load equivalent to the coefficient of static friction between the slide plate and the saddle times the weight on the saddle. In the past, I have also designed the anchor rods for this same load; however, I am now thinking this may be overly conservative. The way the system is designed, the compressive force in the vessel increases as it heats up. It continues to increase until it overcomes the static friction at the sliding plate end, and then the vessel slides, releasing the load. Ultimately, the anchors on the sliding end never see a load from thermal growth because of the slotted hole. Because the load is released on the sliding end, the friction on the fixed end is never overcome. If this friction force is not exceeded, the vessel cannot slide to engage the anchors. Does that mean the thermal expansion never places load into any of the anchors? Essentially, if the vessel was set on the same grout/slide plate configuration with no anchors and no other transient loads, I would anticipate the same exact result. The fixed end would never move, and the sliding end would only move longitudinally with the thermal growth of the vessel. Does this sound correct?






RE: Anchor Rods for Thermal Growth of Horizontal Vessels
Most often, it is more than just friction holding the fixed end - the bolts and bolt holes themselves are designed against shear and for just enough "slop" for construction tolerances.
RE: Anchor Rods for Thermal Growth of Horizontal Vessels
RE: Anchor Rods for Thermal Growth of Horizontal Vessels
RE: Anchor Rods for Thermal Growth of Horizontal Vessels
RE: Anchor Rods for Thermal Growth of Horizontal Vessels
RE: Anchor Rods for Thermal Growth of Horizontal Vessels
Of course, if you have a wind or seismic event that could enough slip. And, the reason for 1/4" oversized holes is that rods are frequently misplaced slightly. Or, you could get some corrosion or something that jams the slide plate (as RACooke suggests). For those reasons, I tend to think it is reasonable to design these rods as if they do see the force. But, I can understand the argument for not doing so.
ASCE has a document on the design of anchor bolts for petrochemical facilities. I don't have a copy, so I don't know if they directly address this subject. But, it looks like section 3.6 (based on the table of contents) might address it. It's probably worth obtaining if you want a more definitive answer from a source you can reference. Give yourself a bit more ammunition if a client or plan checker or peer review ever question the decision.
RE: Anchor Rods for Thermal Growth of Horizontal Vessels