"The airframe was not designed to handle the load from the winglets...the winglets came later..."
The outer wing was modified with different structure parts before the winglets were installed on the post factory retro-fit NG aircraft.
"For example, a BBJ retrofit, accomplished according to an FAA supplemental-type certificate, involves the following tasks. (This listing does not constitute a complete work instruction package.)"
- Removal and replacement of the outboard upper and lower skin panels .
- Removal and replacement of rib 25, which is third from the outermost rib ).
- Installation of stiffeners across rib 25.
- Cutting of the closure rib (rib 27) and trimming of the two spars
- Installation of the new center section of rib 27 and the new winglet attach fitting
- Installation of the spar attach fittings .
- Installation of the aft-position light.
- Installation of the winglet ."
"For airplanes in production, the wings are strengthened throughout the wingbox to accommodate the winglet loads with full use of the speed brakes to the in-flight detent position. The in-production modification meets the same design criteria as those for the retrofit. However, during production, structural strengthening is accomplished by increasing the gage of spars, stringers, ribs, and panels. Rib 27 incorporates bolt hole patterns that allow attachment of either a winglet or a standard wingtip. The winglet is installed in final assembly."
http://www.boeing.com/commercial/aer...let_story.html