I was wondering can a post be skipped when we install longer posts at half post spacing?
There would be a couple of concerns about omitting a post within the 7-ft long post, half-post spacing MGS adjacent to slopes: 1) increased rail tension associated with pocketing and stiff regions on both sides of the omitted post and 2) vehicle snag on the downstream posts. The 7-ft posts at half-post spacing resulted in a stiffer/stronger system than standard MGS. Typical MGS deflections for MASH TL-3 are around 45 – 50 inches. This 7-ft post with half-post spacing saw a maximum deflection of around 27 inches – near the deflections observed for MGS behind 6-in. tall curb (increased embedment depth from soil backfill) . Recall, when a post was omitted within the MGS adjacent to curb system, the system failed due to rail rupture. Subsequently the rail was nested around the location of the omitted post and the modified system passed MASH TL-3. Thus, we would recommend using nested rail around any omitted post location within your 7-ft post at half-post spacing MGS to mitigate the risk of rail rupture. I believe the MGS with curb system used 37.5-ft of nest rail. You could conservatively use the same length and implementation recommendations as the MGS with curb and omitted post for your situation.
Potential for vehicle snag is difficult to quantify. We don’t think omitting a single post within this type of MGS installation should cause snag issues. The half-post spacing on both sides of the omitted post is likely enough to hold vehicle-post interactions similar to that observed during the test. However, we can’t know for sure without further evaluation.