Step 1: Set up the throat section of a fillet weld.
For a fillet weld with leg size \(h\), the throat is the diagonal weakest plane of the weld cross section, and its thickness is \(t = h\cos 45^{\circ} = \dfrac{h}{\sqrt{2}}\).
Step 2: Find the throat area of one weld run.
Each weld run has length \(l\), so the throat area of one weld is \(A_t = t \times l = \dfrac{hl}{\sqrt{2}}\).
Step 3: Account for both welds carrying the load.
The figure shows two parallel fillet welds, one on top and one on the bottom of the plate, both carrying the load \(P\) together in shear along their throat. Total throat area \(= 2 \times \dfrac{hl}{\sqrt{2}} = \sqrt{2}hl\).
Step 4: Apply the allowable shear strength condition.
At the critical load the shear stress on the throat reaches \(\tau_{all}\), so \(\tau_{all} = \dfrac{P}{\sqrt{2}hl}\), giving \(P = \sqrt{2}hl\tau_{all}\).
Final Answer:
The critical load the joint can carry is \(\sqrt{2}hl\tau_{all}\).
\[ \boxed{P_{critical} = \sqrt{2}\,h\,l\,\tau_{all}} \]