Last month, we discussed the importance of bracing and the effect that bracing has on the strength and stability of a scaffold. We also discussed the effects the brace connection to the scaffold leg have on the strength of the overall scaffold. Finally, we discussed that there are various types of bracing, including cross-bracing, diagonal bracing, tie bracing to an existing substantial structure, and horizontal bracing, that can be used to brace scaffolds. This month we’ll discuss bracing variations for frame scaffolds.
As many scaffold users and erectors are aware, a frame scaffold must have some sort of cross bracing in order to be stable. Figure 1 shows the most basic type of frame scaffold tower that can be erected. While only one side of the tower is shown, this cross bracing is on both the front and the back of the scaffold. (For discussion purposes, the front of the scaffold is the side closest to the face of the structure.) The cross braces are attached to the studs of the scaffold frame leg, and the cross brace members are attached to each other where the members cross each other. This connection is critical since the attachment of the brace to the leg usually gives no bending strength because the brace normally can rotate on the pin. The First Rule of Scaffold Bracing can be developed from Figure 1.
“Each scaffold leg must be braced to at least one other leg and/or to an adjacent properly braced structure.”
Note that the rule states that each leg must be braced, not necessarily cross-braced. In the case of Figure 1, cross braces have been used and this tower meets the requirements of the First Rule. Figure 2, consisting of several independent towers also meets the requirements of the rule. Figure 3, which uses horizontal rails to connect the independent towers of Figure 2 together, also obeys the rule. What is the advantage of Figure 3 over Figure 2? Since horizontal ties are required on tall scaffolds at each end of a scaffold and every thirty feet in between, each tower of Figure 2 would be considered an independent scaffold, requiring ties. The scaffold of Figure 3 would be considered one scaffold since all frames are connected to each other, thus reducing the number of horizontally located ties. Figure 4 is another alternative that meets the requirements of the first rule and reduces the number of horizontal ties. Continuous cross bracing is desirable, as illustrated in Figure 5, since it results in a more rigid structure. This also allows for the removal of selected braces without jeopardizing the structural integrity of the scaffold. Remember that the cross braces in the end bays can never be removed since the end leg will lose its bracing to another scaffold leg, violating the First Rule of Scaffold Bracing.
Other bracing alternatives can be used when access to the face of the work is required. Figure 6 illustrates a scaffold that is properly braced, complies with the First Rule of Scaffold Bracing, provides reasonable stiffness to the scaffold, and allows for open (non-cross braced bays) in alternate bays. This bracing scheme can be used on both the front and the back of the scaffold. Another method, used primarily in the western United States, uses a mixture of cross, horizontal braces called goosers, and additional ties to the structure. Figure 7 illustrates this method. Cross braces are used on the end bays and every third or fourth bay horizontally. The horizontal goosers are installed on the top ledger (horizontal member) of each frame, at the front leg. Ties to the structure are installed every twenty feet horizontally (every other leg), and every thirteen feet vertically (every second frame). In this type of installation, the ties to the structure must not only take tension and compression loads but select ties must also be installed so they can resist sidesway loads of the scaffold legs. It is important to note that this design will result in a lower allowable leg load than what is published in manufacturers’ leg load charts due to the greater spacing between braces. While it is recommended that continuous cross bracing be used on the back of the scaffold, the gooser/cross brace combination is often used on the back leg too, further decreasing the capacity of the scaffold.
Other patterns of bracing may be used, provided the scaffold has been designed by a qualified person. If the First Rule of Scaffold Bracing is applied, any problems caused by insufficient bracing will be minimized. Some things to remember about bracing frame scaffolds include:
1. Just because you’ve “always done it this way,” doesn’t necessarily mean its right.
2. Just because the scaffold didn’t fall over when a brace was removed doesn’t mean the brace isn’t needed.
3. Don’t underestimate the importance of the ties to the adjacent structure.
4. Don’t overestimate the strength of the adjacent structure.
5. Removal of critical cross braces and ties solely because “they’re in the way” is really stupid.
6. Don’t tamper with the bracing if you don’t understand it.
7. A qualified person is required for all scaffolds.
8. If in doubt, cross brace the front and back legs on all bays.
9. It is unlawful to erect, dismantle, or alter a scaffold except under the supervision of a competent person, qualified in scaffold design.
Proper bracing of any scaffold is directly related to the strength and stability of the scaffold; don’t let your next scaffold be a bracing statistic. Remember, “Each scaffold leg must be braced to at least one other leg and/or to an adjacent properly braced structure.”