In Eurocode, two equations for loads are given for STR limit state:
Designer is required to calculate using both, and the design value of the load comes from the equation that gives the least favourable result. But what is the actual difference of these equations, why do we need two? I can see that the bottom equation (6.10a) only takes into account permanent (dead) loads (letter $G$) and the one above considers temporary (live) loads as well (letter $Q$).
Why don't we need to consider the live loads in one of the equations? My lecture material says that the lower equation is rarely the deciding one, but it must always be checked. I've read my lecture materials and the Eurocode section on this, but I can't really get any good explanation of this. If somebody could explain the difference between the two equations and why we need them both I would be very grateful!