Les answered Ashleigh as follows:
The thing about lintels is that they are seen by builders as a cheap solution to a problem and they often use them incorrectly, mainly because the thought of constructing a reinforced concrete beam is just too much hassle for them.
The first point of comment is that pre-stressed lintels do work and have a purpose BUT they need to be used in the correct places and be properly installed. The second point is that there needs to be at least 600 mm of brickwork above the lintel with brickforce between each layer and the lintel needs to be at least 400 mm longer to each side than the gap that it is spanning. This over-run has the effect of holding the ends of the lintel down to minimise the chances of producing the little 45 degree crack that one so often sees at the top corner of the window when the mid-part of the lintel deflects under the load above it.
In certain instances, it is prudent to have a double row of lintels with some reinforcing bars placed between the two layers. This, however, depends on the span it is covering, the amount of brickwork above it, and whether it is supporting the weight of a roof or not. A structural engineer can advise you there.
If a sliding door is being fitted and the lintel has not been correctly designed or fitted then the chances are quite real that it will deflect in time and could jam the sliding door or produce cracks at the top corners of the opening that will never go away.
As a matter of personal preference, I tend to specify reinforced concrete beams for any span over 3,5 metres, or even less depending what loading is being imposed above it. I do not really think that cost-effectiveness really comes into the picture when looking only at lintels over long spans in the sense that it is a matter of design, correct application and correct installation. There will always be a point where the degree of deflection where lintels are used will be unacceptable and where a concrete beam should be used instead but this is within the realm of a structural engineer.
A bit of a long answer but I hope that it answers your question.
If you need any further assistance please give me a call.