Expert: Arshad Khan - 4/7/2009

Question
QUESTION:  precast people refused to design slabs to accommodate for the window cleaning equipment load claiming inadequacy of 240mm thk slabs, now I will design a cast-in situ slab but have no idea how to deal with the load coming from the window cleaning equipment, the supplier gave reactions on all four wheels of the window cleaning equipment and the working positions i.e. when perpendicular and when inclined, the precast people increased all reaction by 2.5(dynamic factor and designed their H.C.S but treated these loads as concentrated on the slab, I AM REALLY UNDER PRESUURE AS THIS IS ON HOLD AND THE WORK IS ALL STOPPING ON IT could you please advise me on this matter.

ANSWER: Hi Amal,

You need to treat this like a moving load for bridge design- presumably the wheels will be running on rails, so you can spread the load a bit rather than taking the wheel laods as point loads- one possibility (depending on the wheel spacing), is to consider the load on two wheels as a UDL equal to total weight on 2 wheels divided by the diatance between them (so, kN/m line load), for which you can design either a beam or a slab.

---------- FOLLOW-UP ----------

QUESTION: i HAVE DONE WHAT YOU ADVISED ME TO BUT NOW i AM FACED WITH A DIFFERENT ISSUE,THE WINDOW CLEANING SUPPLIERS PROPOSED A SYSTEM WHERE THE  CRADLE RUNS ON A 150MM THICK CONCRETE PAD ON TOP OF THE ROOF SLAB,SHOULD THIS BE DESIGNED AS PART OF THE SLAB BEARING IN MIND THAT THE RAILING OF THE WINDOW CRADLE IS BEING DIRECTLY FIXED TO THIS CONCRETE PAD SO ALL THE LOADING IS ACTUALLY APPLIED TO THE PAD WHICH IS SITTING ON THE ROOF SLAB,WHAT ABOUT WATER PROOFING WHERE SHOULD IT COME IN THE PICTURE.

Answer
Hi Amal,

In this case you need to design the slab to carry patch loads (under footprint of the pad), and you can detail waterproofing to go all around and above these pads. They should not designed as part of the slab, but considered as part of the dead load.