Are you creating a destroyed city but want other effects than markers and craters? This guide will help you with setting a building on fire at the start of the game.
Select the building you want to set on fire. You can add some destruction in it but the less hp the faster it will be completely burned down. Smaller buildings will also burn down faster than big buildings.
Press CTRL-G and add a name. I'm following Relics name system by naming it eg_xx because it is an entity group
Save your map but don't export it yet. Go to the folder of your map and open "<yourmap>_id.scar"
Add this under the entity groups:
eg_burninghouse1 = EGroup_CreateIfNotFound("eg_burninghouse1")
Entity_SetOnFire(EGroup_GetSpawnedEntityAt(eg_burninghouse1, 1))
You will have to do this every time between saving your map and exporting since world builder overwrites this every time you save.
Export your map and start the game!
End result:
EDIT:
I forgot to add you can also make the buildings invulernable so they will always keep burning at 100% hp.