Ama-semi trailer ombhede ophansi aklanyelwe ngqo ukuthutha izinto zokusebenza ezinkulu nezisindayo. Lawa ma-trailer anompheme owehlisiwe, ovumela ukuthuthwa okuphephile nokusebenza kahle kwemishini emikhulu, izimoto, nezinto zokwakha. Inzuzo eyinhloko yama-trailer emibhede ephansi yikhono lawo lokuthwala imithwalo edinga imvume eyengeziwe yokuphakama, njengama-excavators, ama-bulldozers, nemishini emikhulu yezimboni. Ukuphakama okuphansi kwedekhi kunciphisa ingozi yokulimala kwempahla, kanye nokunciphisa isikhungo sonke sokudonsela phansi, kunikeze ukuzinza okukhulu ngesikhathi sokuthutha. 

Ama-trailer emibhede ephansi abalulekile ezimbonini ezifana nezokwakha, ezezimayini, nezokuthutha izinto ezisindayo. Zilungele ukuthwala imithwalo emikhulu noma ebanzi futhi zivame ukusetshenziswa kumaphrojekthi lapho imishini emikhulu idinga ukuhanjiswa phakathi kwezindawo zokwakha noma imisebenzi yezimayini. Ukwakhiwa komsebenzi onzima wama-trailer ombhede ophansi kuqinisekisa ukuqina nokuphila isikhathi eside, kanye nekhono lokuthwala imithwalo esindayo kakhulu ngokuphepha emabangeni amade.