Bread dough doesn't keep its shape

I've just made some 50/50 loaves using a biga. The taste is quite good but I'd like the crumb to be more open.

The biggest issue is that while raising the dough tends to "spread" rather than rise. What did I do wrong?

Here is my recipe:

For the biga: 500g manitoba flour 300ml water 8g active dry yeast

After 24h I add: 500g wholemeal flour 300ml water 1tbsp vanilla paste 1tsp sugar

I've shaped the loaves straight away as I know wholemeal bread doesn't like long proofing. I've let the dough raise 1 hour at room temperature and 10 hours in the fridge. This morning I found the dough slightly raised and evenly spread in the pan (more like a focaccia than 3 loaves :-) ). I've reshaped a bit, just separating the three loaves.

In the oven the size hasn't changed much (180 C, with a bit of water to keep humidity right).

Any suggestions on how to improve my bread?