You dont' use parapet walls to hang scaffolding. You have to build a small framework to overcome the parapet height and then cantilever wood or steel beams to hang your scaffolding.
Actually, there is alot of work being performed directly from parapet clamps hanging on the parapet itself. I was looking into how to properly identify the loads imposed by the 1,000 pound scaffold setup.
VBI:
It seems to me that you have to know the construction details of the individual parapet, because the capacity will vary from one type to another, in terms of structural loadings, tensile, compressive and moment capacities. One would assume that most parapets could take some compressive loading from a scaffolding hook if it is distributed over some length of the parapet. But, this depends upon how the parapet is capped also. Is the cap glazed clay tile, bent sheet metal on wood blocking, etc? These might be crushed or dented by the hook loads. The way the hook imparts its load should probably not induce tension on one side of the wall. In other words the scaffolding loading should not produce bending in the parapet wall. Then OSHA comes into play as to factors of safety of these support devices or systems.
Chicopee’s method of cantilevered beams with cribbing or front frame/legs near the parapet and the back span counter weighted eliminates concerns about the condition of the parapet and the damage you might do to them. And, you should review OSHA on this arrangement too. Also, much of what you see being done, is being done by some very brave (naive?) works with very little engineering input, until some scaffolding falls.
Thanks, the reality is that hooks are being thrown on parapets all over the city. Until failures start occuring, this will be an aceptable approach. Parapet clamps are not permitted because they do create a leverage and impose bending on the structure. I was wondering if you folks new about any resources to properly analyze this condition. I'm not so sure I understand what you mean about cribbing. Thanks again!
Cribbing is to place dunnage (wood blocks or beams) in certain areas where the underlying material is weak. For example the clay caps on parapets are brittle and weak in tension, so you would want to protect them with wood material so as not to chip away or crack the clay.
I would not do something like this just because it is being done elsewhere in the city. I had the unfortunate experience of looking at a failed awning (not designed by me) and giving an opinion a few years ago in a community because it randomly fell off and killed someone. May not be the exact same situation, but it sounds similar. The awning was attached only to the building facade and not to the structure in this case.
AGREED! But certainly hooks can and should be an option in some cases. I think there needs to be a standard or uniform way to evaluate the loading and wall conditions. I come up empty in my research on the subject however.
Having designed a few parapets, I have NEVER designed one to take the kinds of loads imposed by a swing stage. Parapet hooks should be tied back to prevent outward tension on the parapet, so theoretically, the parapet only sees a vertical down force. It should be simple trigonometry at that point. For tiebacks, the lifeline anchorages are 5000 lbs capacity per person, and cannot be combined with the swing stage tiebacks, for obvious reasons.
(I especially like the part about "Does this parapet look capable of supporting at least 4X the load imposed on it by the cornice hook?" Does it LOOK like? Really?)
Agreed. I would assume 0 load on the tie back and only the vertical downward load of 1000#. That is why there are few failures. Certainly most parapets could support a purely vertical compressive load of this magnitude. But that is funny. Does it LOOK like it can? No disrespect to window washers but...not sure they are the most qualified to say.
