INVESTIGATION OF WITHDRAWAL RESISTANCE OF LAG SCREWS FROM BANGKIRAY WOODS

Abstract

Lag screws are commonly used in knock down constructions where screws
is not possible. The resistance of the lag screws is provided by friction between
threaded parts and wood. This paper presents experimental investigation of
withdrawal strength of lag screws embedded into Bangkiray woods. The objectives of
the investigation were to determine the behavior of embedded screws subjected to
withdrawal forces.
To meet the experimental objectives, three specimens of an 8 mm diameter lag screw
embedded into a 80 x 150 x 600 mm bangkiray woods were assembled and tested to
failure. The lag screws were 100 mm long with threaded length of 60 mm (effective
length of 55 mm). However, in this experiment, only 40 mm of the threaded part was
penetrated into the main wood and giving the effective penetration length of 35 mm.
Direction of the screw was perpendicular to wood grains. Two equal point loads were
applied on both sides of the screws through two pieces of 50 x 70 x 500 mm bangkiray
woods connected using screws. The loads were increased incrementally every 1 kN or
1kN/minute. Axial deformations of the screws were measured using a dial gauge
located at the center of the lag screws for every load increment.
The results show that failure modes occurred due to failure of the woods which were
bearing failure of side beams and pull-out of the screws due to lose of main beam
resistence. These types of failure were mainly dictated by the absence of washers
between the bearing wood and the head of the screws. Slip of the embedded part at
maximum load is about 9.25% and 10.93% of the embedded length. The average
maximum withdrawal strength of the screw embedded 40 mm into bangkiray wood is
12.0 kN, excluding data of specimen S1. Prediction of withdrawal strength for
embedment length of 60 mm is 24.10 kN.