Access holes through the web of a built-up member or a rolled member is to provide sufficient access for the welder to deposit sound weld across the width of the flange. Weld such as that described can be found in moment connections between a column flange and the top and bottom flanges of a beam or girder.
In the case of welding a bottom beam flange to the column flange, the welding is done from the top side in most cases. The welder has to extend the welding electrode through the access hole and progress to the outer free edge of the flange. The backing bar, if used, is typically placed under the groove to support the root bead while it is being deposited.
An alternate to welding the lower beam flange from the top side is to weld it from the bottom side. This is not usually done because of the difficulty in depositing weld in the overhead position. However, there are situations where overhead welding cannot be avoided. In this case the backing bar, if used, is placed on the top side of the lower flange and the backing bar passes through the access hole to provide continuous backing for the full width of the flange.
The attached sketch depicts the two cases I have mentioned in my response. Shown in the sketches are the lower beam flange welded to the column flange. The web connection shown is bolted.
In the event the detailer decides the entire connection should be welded, i.e., the top and bottom flanges welded to the column flange, as well as the web to the column flange, the access hole would still be used to interrupt the weld joining the web to column and the web to beam flanges because it is not a good idea to have welds intersecting at 90 degrees or other angles. The residual stresses resulting from the welding operation are added as vectors. The resultant can easily exceed the ultimate tensile strength of the filler metal, base metal, or both. The concern with residual stresses becomes a greater issue if higher strength steels are involved and when there are more than two intersecting welds. As an example, consider the beam as a built up member where the web is welded to both the upper and lower flanges. Then consider the welding of the lower flange to the column flange. Then consider the beam web welded continuously to the column flange without the use of the access hole. The connection would have three intersecting welds, each perpendicular to the other. Disregarding ductility, because in a practical sense the joint is highly restrained, the resultant force developed because of the three welds will most certainly exceed the UTS of the base metal and the weld if matching filler metal is used. Increasing the strength of the filler metal makes the problem worse. If anything, under matching the strength of the filler metal (relative to the strength of the base metal) would result in the only reasonable hope of preventing a crack from forming.
Consider ASTM A36 welded with E70XX filler metal: The resultant from the residual forces would be on the order of 51 ksi with two intersecting welds.
The resultant force would be on the order of 62 ksi for three intersecting welds, which exceeds the min. UTS or 58 ksi.
If ASTM A992 is used the resultant for two intersecting welds would be 71 ksi and three intersecting welds would be 87 ksi. Both cases exceed the UTS of the base metal (65 ksi min.)
So, as you can see, the assess hole serves several useful purposes.
Best regards - Al
