There shall be no permanent opening in any sidewalk, extending beyond the property line, except such openings as are herein expressly authorized. There may be an opening, providing it is covered with metal sidewalk trap doors. The outer edge of the doors shall be within two feet of the outside edge of the sidewalk curb, the inner edge of which shall not be more than seven feet from the outer edge of the sidewalk curb. Every door shall be provided with metal guards for the protection of the public when open and shall be flush with the sidewalks when closed, including hinges. There may be openings through sidewalks for the admittance of fuel. All openings in sidewalks shall have metal or cement covers flush with the walk, and their upper surfaces shall be roughened. All parts of sidewalks, including illuminating tile or other structural parts, shall be capable of sustaining a load at all points simultaneously equal to 400 pounds for each superficial foot of the surface thereof. There shall be no permanent openings in the form of gratings of any description in any sidewalk.