Yes, there is RAW on PHB 175 about help.
However, for situations outside the norm, you can decide with general rules and common sense whether it's a 1 or 1.5 person job, or if you can actually get everyone involved.
DMG page 5:
The rules don't account for every possible situation that might arise during a typical D&D session.
DMG Page 246:
When characters need to saw through ropes, shatter a window, or smash a vampire's coffin, the only hard and fast rule is this: given enough time and the right tools, characters can destroy any destructible object. Use common sense when determining a character's success at damagin an object.
When you need to reference "common sense" things, you can reward players for using real-world physics such as a lever to multiply effectiveness, or use the lifting/carrying rules:
PHB page 176:
Your carrying capacity is your strength score multiplied by 15
You can push, drag, or lift a weight in pounds up to twice your carrying capacity (30 times strength score)
Larger creatures can bear more weight, whereas tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag or lift. For a tiny creature, halve these weights.
So it will really depend on the given task.
For the example of the immovable rod:
So here is a link to a lever force calculator: http://www.engineersedge.com/calculators/levers/page_levers_1.htm
Basically, if the target amount of force is 8000 pounds (W in this link), and we were able to get a lever underneath the object and W is length (x) 50 inches from a fulcrum, (L) length from fulcrum is where the player/party must exert force. If they can exert 500 pounds of force, they will "break" the rod's 8000 pound limit. So with a fulcrum object that won't be crushed under the load, and a lever object that also won't break under the load, a lever and fulcrum (proper tools) could make this pretty trivial if you have room to maneuver a lever/fulcrum in such a way as to get appropriate force.