A guardrail's top rail must be able to withstand at least 200 LBS of force.
"Guardrail systems are capable of withstanding, without failure, a force of at least 200 pounds (890 N) applied in a downward or outward direction within 2 inches (5 cm) of the top edge, at any point along the top rail." - OSHA Requirements for Guardrail and Safety Railing Compliance
The above statement is where I based my answer of minimum force of 200 pounds.