Increment decimal value with active record

[eluser]Acidrain[/eluser]
Hey,

I can't get it to work.
The value in the database does not increment.

Code:

$myFloat = 45.83;
$id = 4;

$editData = array(
    'column_name' => 'column_name + ' . $myFloat
);

$this->db->where('id_column', $id);
$this->db->update('table_name', $editData);

Am I doing something wrong?

column is DECIMAL(20,5)