Department of Applied MechanicsSardar Vallabhbhai National Institute of TechnologySurat - 395007

Block Shear Failure in Tension

Members

Prepared By: Guided By: Prakash Agarwal Prof A J Shah

Linear members in which axial forces act so as to elongate the member.

Tension members carry loads most efficiently, since the entire cross-section is subjected to uniform stress.

Since axially loaded tension members are subjected to uniform tensile stress, their load deformation behaviour is similar to the corresponding basic material stress strain behaviour.

Tension Members

Tension Members (Contd.)

Design Strength of Tension Members

The strength of tension members is the minimum of the

following three categories as stated below:

Yielding of the gross section

Fracture of the effective net section

Block shear rupture

Block Shear Failure Mechanism

In block shear mode, the failure of the member occurs along a path involving tension on one plane and shear on a perpendicular plane along the fasteners.

Block Shear Failure Mechanism (Contd.)

When a tensile load applied to a particular connection is increased, the fracture strength of the weaker plane approaches.

The load can be increased until the fracture strength of the stronger plane is reached and during this time, the weaker plane yields.

The total strength of the connection equals the fracture strength of the stronger plane plus the yield strength of the weaker plane.

Block Shear Failure Mechanism (Contd.)

IS 800: 2007 assumes that when one plane, either tension or shear, reaches ultimate strength the other plane develops full yield.

[1] It is assumed that failure load is reached when rupture occurs along the net tension plane and full yield is developed along the gross shear plane.

[2] Rupture occurs along the net shear plane while full yield is developed at the gross tension plane.

Design Strength due to Block Shear The block shear strength, Tdb of a connection is

taken as the smaller of,

Tdb = Avg fy / √3 γm0 + 0.9 Atn fu / γm1

ORTdb = 0.9 Avn fu / √3 γm1 + Atg fy / γm0

where, Avg and Avn = minimum gross and net area in shear

along a line of transmitted force respectively, and

Atg and Atn = minimum gross and net area in tension

the hole to the toe of the angle perpendicular to the line

of force respectively.

Block Shear Failure Geometry

Block Shear Rupture in a Gusset Plate

The model used in AISC 2005a assumes that block shear failure occurs by rupture (fracture) on the shear area and rupture on the tension area.

Rn = 0.6 fu Anv + Ubs fu Ant ≤ 0.6 fy Agv + Ubs fu Ant

where Ubs = 1.0 when the tension stress is uniform (angles,

gusset plates and most coped beams), and Ubs = 0.5

when the tension stress is non-uniform.

AISC 2005 Specification for Block Shear

In Eurocode 3 pr-EN 1993-1-8: 20xx, the design value for block shear (termed “block tearing” in the standard) for symmetric bolt groups, is determined from the equation:

Veff,1,Rd = fu Ant / γM2 + (1 / √3) fy Anv / γM0

(under centric loading)

Veff,2,Rd = 0.5 fu Anv / γM2 + (1 / √3) fy Ant / γM0

(under eccentric loading)

Eurocode 3 Specification for Block Tearing

Investigated block shear tear out (TO) failure in gusset-plate welded connections in both very high strength (VHS) tubes and structural steel hollow sections (SSHS).

The existing design rules for TO failure from the American, Canadian and European standards were examined by comparing their predictions with both test results from welded connections in VHS tubes and previous tests on similar connections to SSHS.

The results suggested that the existing design rules were not adequate to predict the connection strength. Therefore, five modifications to the existing design rules were examined.

Study of Ling et al. (2006)

This study reported on the experimental testing of 11 welded gusset plate specimens which were subjected to tension, and ultimately to block shear failure.

Finite element analyses were conducted to predict the failure loads of the specimens and the respective load-carrying capacities of the shear and tension planes.

He concluded that the tension plane in the welded connection details could develop stresses in excess of the ultimate tensile strength due to the presence of stress triaxiality. Finally, design recommendations were given based on the experimental and numerical findings.

Study of Topkaya (2004)

Their study examined the block shear capacity of steel angles (single as well as double angles), for bolt holes in one or more rows, and with staggered and non-staggered holes.

The area as per their improved approach was termed as effective block gross shear area and was somewhat less than the block gross shear area, as per current practice.

There was a considerable improvement in values of professional factors when the concept of effective block gross shear area was used in the computations.

Study of Gupta and Gupta (2004)

The goal of their study was to accurately model structural tee specimens using the finite element method in order to predict the failure mode.

The finite element models were accurate in predicting failure mode, especially when compared to the results of the physical tests, and normalized failure load plots showed similar shapes.

In terms of behavior, it was noted that an eccentric tension member was subjected not only to an axial force but also to a moment due to the connection eccentricity.

Study of Epstein and McGinnis (2000)

An ISMB 600 is connected to a column by web cleats with a single row of bolts. The applied reaction is 350 kN and there are four 20-mm-diameter bolts through the web as shown in the following figure.

Numerical Example

Block shear strength using different codal provisions:

IS 800: 2007: 466.32 kNAISC 2005: 484.56 kNEurocode 3: 367.7 kN

Hence, the design block shear strength for different codes arranged in increasing order is:

Eurocode 3 < IS 800:2007 < AISC 2005 (LRFD value)i.e. 367.7 kN < 466.32 kN < 484.56 kN

Thus, all the three standards give the design block shear strength which is greater than the applied reaction of 350 kN.

Numerical Example (Contd.)

Block shear failure - An important limit state

Inconsistencies in various design standards used for calculating block shear strength.

Stress concentrations, connection geometries and other factors that may contribute to yield or rupture of the tension plane without affecting the shear plane can also be expected to reduce block shear load capacity.

Equations having high safety indices than the traditional target for connections can result in uneconomical connections.

Conclusion

Epstein, H.I. and McGinnis M.J., “Finite element modeling of block shear in structural tees”, Computers and Structures, vol. 77, pp. 571 – 582, 2000.

Eurocode 3 , prEN 1993-1-8: 20xx, “Design of steel structures” (English version), European Committee for Standardisation, Brussels, 26 Feb. 2002.

Gupta, M and Gupta, L.M., “An improved approach to compute block shear capacity of steel angles”, Connections in Steel Structures V – Amsterdam, pp. 347 – 354, 2004.

Institute for Steel Development & Growth, “Chapter 5 - Design of Tension Members” version II, (http://www.steel-insdag.org/new/pdfs/Chapter5.pdf Accessed Nov. 20, 2009).

IS 800:2007, “General Construction in Steel – Code of Practice”, Third Revision, Bureau of Indian Standards, New Delhi.

References

Ling, T.W., Zhao, X.L., Al-Mahaidi, R. and Packer, J.A., “Investigation of block shear tearout failure in gusset-plate welded connections in structural steel hollow sections and very high strength tubes”, Engineering Structures, vol. 29, pp. 469 – 482, 2007.

Segui, T. William, “Design of Steel Structures”, Cengage Learning India Private Limited, New Delhi, 2009.

Subramanian, N., “Code of Practice on Steel Structures - A Review of IS 800: 2007”, Civil Engineering & Construction Review, pp. 114 – 134, August 2008

Subramanian, N., “Design of Steel Structures”, Oxford University Press, New Delhi, 2008.

Topkaya, C., “Block shear failure of gusset plates with welded connections”, Engineering Structures, vol. 29, pp. 11 – 20, 2004.

References (Contd.)

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