Does anyone know how much STR is Equal to one weapon attack?

Well, Locked is somewhat justified in his reply. The question placed cannot be answered because of the way the stats work. One point in weapon attack does not give a predetermined amount of damage to the player's range, and neither does one point in strength. In other words, no amount of strength is ever "equal" to one point of weapon attack.

If you need to know how much strength or weapon attack will affect your damage, here's the table from Fiel's formula thread that lists out the damage formulas:

Weapon Type	Base Mastery	Max Damage Formula
1H Sword	20%	1.20*(4 * STR + DEX) * (Weapon Attack / 100)
1H Axe	20%	1.20*(4 * STR + DEX) * (Weapon Attack / 100)
1H BW	20%	1.20*(4 * STR + DEX) * (Weapon Attack / 100)
Dagger	20%	1.30*(4 * LUK + STR + DEX) * (Weapon Attack / 100)
Wand	25%	1.00*(4 * INT + LUK) * (Magic Attack / 100)
Staff	25%	1.00*(4 * INT + LUK) * (Magic Attack / 100)
Barehands	20%	1.43*(4 * STR + DEX) * (Weapon Attack / 100)
2H Sword	20%	1.32*(4 * STR + DEX) * (Weapon Attack / 100)
2H Axe	20%	1.32*(4 * STR + DEX) * (Weapon Attack / 100)
2H BW	20%	1.32*(4 * STR + DEX) * (Weapon Attack / 100)
Spear	20%	1.49*(4 * STR + DEX) * (Weapon Attack / 100)
Polearm	20%	1.49*(4 * STR + DEX) * (Weapon Attack / 100)
Bow	15%	1.20*(4 * DEX + STR) * (Weapon Attack / 100)
Crossbow	15%	1.35*(4 * DEX + STR) * (Weapon Attack / 100)
Claw	15%	1.75*(4 * LUK + DEX) * (Weapon Attack / 100)
Knuckler	20%	1.70*(4 * STR + DEX) * (Weapon Attack / 100)
Gun	15%	1.50*(4 * DEX + STR) * (Weapon Attack / 100)

Oh, and your base damage is found by multiplying your max damage by your weapon mastery/100. Weapon mastery can be found by adding your weapon's base mastery to the mastery added by your skills