Cross on the ceiling - Is the joist about to give way?

[Chetic]

We have just moved into a split-level house from 1973 with a crawl space and a renovated half-basement. Everything seems fine except for this, which we of course didn't discover until long after inspections, handover, etc.:


It runs through an unusually large and open living room on the lower level. Do we need to put up a support pillar or build walls?

Now we have also decided that the best arrangement is to move up and fill the bookshelves up there. It will be a few hundred kilos. How worried should we be? How can we find out if it will hold without having to test it ourselves?

 

[useless]

What does the picture show? Do you have any drawings of the construction and floor plan?
It's not impossible that a previous owner removed a load-bearing wall to get a larger room...

 

[Chetic]

The image shows the ceiling view from the large living room in the basement with the crack that goes like a cross in the ceiling.
I have no blueprints. How do you obtain such?

 

[useless]

Drawings might be available at the municipality if you're lucky.
What has cracked? Is it just the paint on the ceiling? How is the floor structure constructed? Wood, concrete?

 

[Chetic]

Wow great tips. Found the drawings from 1972 on the municipality's website! The living room is 28.5m² and it looks the same in the drawing as it does today.
You can only see a crack in the paint, but that's a sign of change, right? If no one has loaded the floor as much as we plan to, it doesn't feel completely safe.
The framework is made of wood. It sounds quite hollow when you walk on it, but probably at a normal level.

 

[prototypen]

Is it the joints on the drywall that have cracked?

Protte

 

[FRS05]

Can't you post the drawing here? Take a picture with your phone and post it here to get good advice.

 

[Chetic]

Are the joints on the drywall that have cracked?

Protte

How do I know that?

Here is the drawing:

https://i.imgur.com/coJ4rVu.png

 

[SysterNett]

Can you draw on the drawing where the cross is?

 

[Chetic]

Looked around a bit and found a very minimal hint of similar cracks (crosses) in other places in the room. The crosses repeat systematically. It could definitely be the seams of the drywall that have cracked! But then how come the cross in the middle of the room where the load is highest is also the most cracked? When should one start to worry?