Technically Yes, but you'll need to check with your building inspector because there are codes regarding structure that vary from place to place, and the inspector isn't going to pass something based on my say so.
The old rule of thumb was 1 inch for 1 foot, so a 12 foot span would require a 2 x 12.
A 4 x 8 isn't equal to a 2 X 16, or even 2 2 X 8's, but it should do if the load isn't too great.
I'm guessing you want to go with 8" stock because you've got some headroom issues. You'd be better off structurally with an LVL (Laminated Veneer Lumber) or OSB (Oriented strand Beam) or a truss, but these are kind of pricey.
If you get in trouble see if they'll let you go with them on 12" centers.
You will have to have an engineer spec that beam for you. You will save yourself time and money by cutting the span with a column.
Even using a 2x12 the spacing for 20 feet will be less than 12 inches on center. I would recommend splitting the distance with a double beam perpendicular to the joist run. This will allow you to go up to 24 inches on center. Your span will only be ten feet aproximately this way. I would double the beam. (2-2x12 joined together) Good luck. when building your beam make sure the laps on the beam are at least 6 foot apart and put a good wood glue on the 2x12s plus for added strength put 1/2 inch O.S.B. in between the 2x12 and I would also lag or bolt them together, at about 16 to 24 inches O.S.B. = Oscilating Strand Board.
250 mm x 400mm
The maximum span for a 2x6 lumber member used as floor joists, spaced 16 inches on center, is typically around 9 feet for residential applications, depending on the load conditions and wood species. For ceiling joists, the span can be slightly longer, usually up to 10 feet. It's important to consult local building codes and a structural engineer for specific requirements and safety factors.
350x450mm with 12-16mm rebars on 5m span
You will have to have an engineer spec that beam for you. You will save yourself time and money by cutting the span with a column.
A double 2x10 beam can typically span up to 12 feet without additional support.
These calculations are not difficult, but you must know the load (including the floor weight) that will be carried. And the deflections that the appropriate building regulations allow.
The maximum span for a 2x6 beam is typically around 10-15 feet, depending on the load it needs to support and the building codes in place.
250 Feet
The recommended pergola beam span for optimal structural support and durability is typically between 12 to 16 feet, depending on the material and design of the pergola. It is important to consult with a structural engineer or contractor to determine the specific beam span needed for your project.
To span a distance of 20 feet, a lumber beam of at least 2x10 size is typically needed for adequate support.
250 Feet
The recommended patio roof beam span according to the patio roof beam span table is typically around 10-12 feet.
250 feet
The recommended pergola beam span according to the pergola beam span chart is typically around 12-16 feet, depending on the specific design and materials used.
A 2x10 beam span chart provides information on the maximum allowable span for a 2x10 beam based on the type of wood and the load it will support. By referring to the chart, you can determine the appropriate span for your project to ensure the beam can safely support the intended load without sagging or failing.