A Review the part A opening width table “Value of Factor X”
The table below can help you determine minimum pier and return widths to confirm the stability of masonry as a way of not requiring structural calculations.
The table can be used as a guarantee that has taken design loads and included a safety factor to ensure it’s stable in all situations.
For instance, if a roof with a maximum 9m span bears onto the wall and the inner masonry leaf is at least 100mm thick and at least a 7.3N/mm² compressive strength, then the X factor is 6. Similarly, if the same structure has a first floor with a maximum 4.5m span onto it then the X factor would still be 6 or if the span is between 4.5m and 6m then the X factor would be 5 instead.
Once you figure out how the X factor is calculated then you can use Diagram 14 to work out the masonry widths:
Here are the simple rules found in Diagram 14 with the main one (point 1) being the opening sizes are limited to 2L/3. So, for instance, if the supporting wall is 6m long then the total of the openings can’t exceed 2×6.5/3 = 4.3m.
The width of the piers can then be designed using the X factor in connection to the centreline of the return or buttressing walls and adjoining openings. If we use 6 as the X factor then we can estimate the pier sizes for a 6m long wall.
The proposed openings work using the formula and X factor with 666mm remaining. Larger openings can still be built however, would require a structural engineer to assess the proposed construction to confirm whether or not additional support is needed by making the walls thicker or by adding wind posts to maintain stability.
Please note, that return corner piers (note 7) still needed to be at least 665mm wide despite the formula offering 330mm for W4. Remember W4 will be built in the wall buttressing the loaded wall so the 665mm brickwork return provides this stability.