Practice Advisory 6: Achieve best practice every step of the way

Inadequate or impractical load paths are provided through diaphragms and transfer diaphragms.

• Review NZS 3101: 2006 Section 13 – Design of Diaphragms.
• Refer general structural concept texts such as: White R et al, Structural Engineering, SESOC Journal, Puzzle Series 2003.

Inadequate consideration of detailing requirements of local effects due to steps in diaphragms, penetrations, or re-entrant corners in diaphragms. Inadequate consideration to confirm primary structural load paths for transfer diaphragms.

• Review NZS 3101 Section 13, note Clause 13.3.3.

Ensure structural analysis allows for accurate determination of the design actions in local discontinuities such as re-entrant corners.

• Refer SAFE slab design software as one means of analysing steps or re-entrant corner in slabs.

New methods and unusual forms of construction can introduce significant uncertainties to structural performance.

A risk-based approach, taking into consideration the likelihood and consequences of an incorrect design, could be used to set the level of specialist expertise, scale modelling, peer review, proof testing and other steps needed to ensure the completed structure will satisfy the design intention.

Inadequate consideration of the detailing requirements for curtailment of reinforcement in tension zones and bond stress in areas of high shear.

Review NZS 3101 Concrete Structures Standard Section 8, Clause 8.6.12.4.

• Refer Park R, Paulay T, Reinforced Concrete Structures, Chapter 13 The Art of Detailing.

Spiral and hoop reinforcing in columns and piles is not being correctly anchored into core concrete in some circumstances.

• Refer NZS 3101 Section 8.

Rebending reinforcing on site can seriously affect the strength and ductility of the bar.

Give consideration to design details that avoid the requirement for rebending.

If it is unavoidable, then it must be done in accordance with manufacturers’ recommendations.

• Covered in NZS 3101 Section 5, Clause 5.3.2.8.
• Also NZS 3109 Concrete Construction, Section 3 Clauses 3.3.4 to 3.3.6.
• Refer to MBIE's report on Grade 500E steel reinforcement.

Poor consideration of the lateral bursting forces associated with cranked reinforcing bars.

Welding of high strength (Grade 8.8 and above) bolts will seriously affect their ductility.

Bolts can be fixed in position through the use of temporary frames or adhesive. See details in HERA Report R4-58.

Shear design of:

• beams supporting precast flooring
• shell beams

may be carried out using unconservative values for the contributory width.

• Review NZS 3101 Part 1 Section 18, Clause 18.5.6.

Note that the width to be used in determining shear capacity will vary as a function of the location along the beam (plastic hinge zone vs. non-hinge zone).

• Refer Bull DK,1984. Park R Behaviour of structural concrete frames with precast concrete shell beams subjected to seismic loading.

Insufficient consideration of uneven loading arrangements or imposed lateral displacements, especially when checking punching shear on cast in-situ slabs/column heads.

• Refer NZS 3101 Commentary Clauses C12.7.1 and C12.7.7.7.3.

Precast concrete is not covered adequately in NZS 3101. A view that precast concrete hollow-core floors performed poorly in recent earthquakes in California.

The performance of precast concrete hollow-core floors in California is not directly comparable to New Zealand situations due to different methods of design and construction.

• Refer CAE Guidelines for the Use of Structural Concrete in Buildings, University of Canterbury, Christchurch, New Zealand. 2nd Edition December 1999.

Uncontrolled saw cuts in diaphragm slabs reduce transfer capacity through reducing concrete depth and potentially cutting reinforcement.

Indicate position and maximum depth of saw cuts clearly. Warn of dangers of cutting too deep or in a different location.

Specify a concrete mix that minimises shrinkage by minimising water content, maximising aggregate size.

Contractors:

Do not carry out saw cuts in diaphragms (including slab on grade) without obtaining the approval from the designer.

Use good curing practices, refer NZMP 3100: 1999 The Guide to Concrete Construction.

In reinforced concrete construction tie-bars between the column and floor system in one-way frames must be sufficient to provide restraint against column buckling.

Lack of proper column restraint can lead to premature failure of the columns and overall structure.

Design, detail and provide reinforcement in accordance with NZS 3101 Section 18, Clause 18.6.6.

Floor system must be able to provide sufficient restraint to prevent column buckling and separation of the columns and diaphragm. Generally require larger of 5% of maximum axial load in column or 20% column shear from lateral forces.

Structural Steel:

Provide detailing that will ensure the member is restrained to the degree assumed in the design calculations.

• Refer NZS 3404, Section 1.3 and 4, Clause 4.8, Section 5, Clause 5.4, Section 6, Clause 6.7.
• Refer HERA Report R4-92, Restraint Classifications for Beam Member Moment Capacity Determination to NZS 3404: 1997.
• Refer Nethercot L, Lateral stability of steel beams and columns: common cases of restraint, SCI 1992.

Over-estimation of theoretical shear stud capacity in precast floor systems on composite steel beams.

• Refer HERA Report R4-113. Notes from a seminar on composite construction, especially sessions 2.2 and 4.2. Consider the effects of accumulated creep and shrinkage and moment continuity effects on stud capacity.

Performance of composite floor systems. Serviceability considerations (deflection, vibration) may control the size and spacing of support members and slab thicknesses in composite floor systems (insitu-slab on steel deck supported on composite steel beams).

• Refer HERA Report R4-107.