EFFECT OF BOLT DIRECTION ON THE BEARING CAPACITY OF DOUBLE SHEAR CONNECTIONS

The direction of bolts and its effect on the bearing capacity of double shear bolted connections has been examined in this paper. Sixteen specimens with up to four bolts have been tested under tensile load. These specimens have been sorted into two series (L, T) according to the bolt direction with applied loads. The experimental results have clearly indicated that the bearing capacity of connections had been considerably affected by changing the direction of bolts. With tear-out failure, All specimens of L series showed high bearing capacity and good ductility while block shear failure was the distinguishing feature of specimens of T series


INTRODUCTION
In steel structures, high strength bolts are commonly used to form bolted connections. In these connections the load be transferred from main plate to the splices by friction between plates.
In bearing-type connections, the load is assumed to be larger than the frictional resistance caused by tightening the bolts, with the results that the plates slip a little on each other, putting the bolts in shear and bearing. In this study and to focus on the bearing failure, the shearing failure of bolts has been excluded.
With satisfactory end distances and deformation around bolt holes is not a design consideration (that is the deformation > 0.25 in), the bearing strength of bolted connections is determined by AISC as the following equation (AISC Steel construction manual, 2005).

≤ 3 (1)
Where, is the clear distance between the hole edge to the plate end or two hole edges, t refers to the plate thickness, d is the bolt diameter, and represents the steel plate tensile strength. Kim and Yara (1999)

MATERIAL PROPERTIES
Two materials have been used in this research .These are steel plates and high strength bolts

Properties of Plates and Test Samples
To determine the properties of the plates used in this study, 12 samples have been configured  Table 1.

High Strength Bolts
High strength bolts have been used to connect the plates. These bolts have been chosen to make a specimen fail in bearing not in shearing mode, Fig. 2.

Geometry of Specimens
To study the bearing behavior of double shear connections, sixteen specimens have been configured .Three plates, one inner and two outer; of the same thickness for each specimen have been connected by high strength 5/8 in (inches) bolts. To make the bearing failure happen in the inner plate, the thickness of each outer plate has been chosen to be equal to the thickness of the inner one. During the test, the gap between outer plates has been stuffed by a non-welding plate, Fig. 3. To control the type of failure which is bearing failure, dimensions of the plates were variable according to the thickness of plate and numbers of bolts, Table 2. For all specimens, standard holes which are 1/16 in (inches) larger than a bolt diameter according to AISC specifications have been drilled precisely using the CNC machine (AISC manual). The distance between centers of adjacent holes was 2 2/3 times the bolt diameter and the distance between the edge of the plate and the center of the edge hole was 1.5 in. The former distance represents the minimum distance recommended by AISC provisions while the latter is more than the minimum. The specimens have been sorted into two types of series (L and T) according to the direction of holes with the direction of the applied loads, number of bolts and thickness of plates. English alphabets and Arabic numerals have been used in designate these specimens. The first character, L or T in the designation of the specimens refers to the direction of bolts either longitudinal or transverse according to the applied loads, the following first numeral refers to thickness of the plate while the second numeral refers to numbers of bolts used, Table 2.

Load-deformation relationships
In this study, sixteen specimens of double shear bolted connections have been tested under uniaxial tensile load. These specimens have been sorted into two types of series (L and T). L series represents eight specimens with bolts positioned in the parallel direction to the applied load while T series with bolts positioned in the transverse direction. All specimens have been tested in tension in a 600 Kn capacity universal testing machine with a displacement rate of 2 mm/min. Fig. 4 shows the load-deformation curves for both series, while Table 3 lists the values of experimental and theoretical ultimate bearing strength of specimens. The theoretical values have been calculated by equation (1). To study the effect of bolts direction on the bearing capacity of specimens, two curves have been collected in one graph, Fig. 4.

Fig. 4. Load-Deformation Curves of Specimens of Series Land T.
Each two curves represent the behavior of two specimens. Both specimens have the same geometry and number of bolts but with bolts in different directions. Experimentally, it is clear that all specimens of (L) series have achieved nominal ultimate loads (1-44) % more than their peer of specimens series T, Table 3. The same conclusion can be noted but with different ratio to theoretical results. It is worth mentioning that and during the test; all transverse holes have suffered from equal applied stresses due to the equal distance between them and the edge of the plate. These stresses have deformed the plate by enlarging the holes longitudinally, Fig. 5

. As
Kufa Journal of Engineering, Vol. 8, No. 2, 2017 25 result for enlargement the holes which represents bearing failure, perpendicular tensile stresses between transvers holes were believed to have developed first and accompanied with shear stresses along two shear lines parallel to applied load. The perpendicular stresses were higher than the shear stresses so that transverse slots have appeared first and this fracture (slot) even affected upon the behavior of load-deformation curves in necking period after getting ultimate capacity, Fig. 5. With increasing the applied load, the piece of plate confined between the transverse holes and the edge has been ripped out totally. This phenomenon has been noted for all specimens of series (T). This is what is called the block shear failure.  With regard to the specimens of series L, in which the direction of bolts is parallel to the applied load. In additional to the higher bearing capacity that has been achieved, there was an another remarkable merit which has been clearly noted. It is the high ductility, Fig. 3. The applied stresses on holes for each specimen were noted to be unequal, Fig. 7. Because of the phenomena of inequality in stresses, Some nearby points of high stress have reached to the yielding but other far points have not . With increasing the load, the yield spread out to other points causing the specimens to fail but with high resistance and more ductility . The noted failure was tear out for all specimens of series L, Fig. 8.
It was clear that two types of failures distinguished the two series, block shear and tear out failures. These two modes of failure are defined as specimens reach its ultimate strength. The ultimate strength is the maximum strength in the load-displacement curves. Eventually, it is concluded that the bearing capacity of double shear connection might be considerably affected by changing the direction of bolts.

CONCLUSION
In this study, sixteen specimens of double shear bolted connections have been examined under uniaxial tension. These specimens have been sorted into two series (L, T) according to the bolt direction with applied loads. The experimental results have clearly indicated that the bearing capacity of double shear connections might be considerably affected by changing the direction of bolts. With tear-out failure, all specimens of L series showed high bearing capacity and good ductility while block shear failure was the distinguishing feature of specimens of T series.

REFERENCES
ASTM A370-02, 2002, Standard test methods and definitions for mechanical testing of steel products, American Society for Testing Materials.
AISC Steel construction manual, 13 th ed. 2005, American institute of steel American institute of steel construction.