How to Pre-Embed Anchor Bolts When Welding Is Not Allowed

According to specifications and design requirements for anchor bolt embedding, without welding — not even tack welding — is allowed between anchor bolts, locating plates and steel reinforcement bars.

Common practice:

1.Weld or tie the lower sections of anchor bolts into a whole with rebar. Then weld this bolt assembly to the main steel reinforcement. At the top reinforcement layer, connect the anchor bolts to the top steel by either welding or tying.Welding is not permitted per design. If we only use binding wire, the bolts will shift easily during concrete pouring, resulting in poor installation accuracy.

2.Use a single positioning plate. Weld the upper plate to the top rebar and weld the bolts directly to the plate. The bottom part is fixed the same way as Method 1. Even with all these welds, the bolt plumbness still cannot be well controlled during concrete pouring and vibration.

3.The double positioning plate method offers the highest installation accuracy. The procedure is the same as our previously published anchor bolt cast-in-place technique.However, some projects strictly follow codes and design requirements. No welding on anchor bolts is allowed, which makes cast-in-place quite difficult. Securing with binding wire provides insufficient rigidity, so positional accuracy cannot be maintained well during concrete pouring and vibration.

To solve the above problems, we can adopt the following construction method:

1.We use double locating plates. Weld the lower plate to the top rebar, but do not weld the anchor bolts; just let them pass through the plate holes.After setting the elevation of the upper locating plate, weld rebar between the four corners of the upper and lower plates to make them a solid unit.Fasten the anchor bolts to the upper plate with double nuts.Weld diagonal rebar braces around the upper plate to the top reinforcement for extra stability, so the bolts won't shift during concrete pouring and vibration.

2.The threaded length of the anchor bolt shall be 350 mm during fabrication.We install upper and lower locating plates. After adjusting the elevation, screw the nuts down onto the top face of the lower plate.Then fit the upper plate, and clamp it tightly with double nuts at the top of the bolt. After aligning the center line, weld the lower plate firmly to the top rebar. Next adjust the verticality of all anchor bolts. Once aligned, weld the bottom nuts to the positioning plate for reinforcement. This finishes the installation.It keeps high installation accuracy and complies with design rules: we never weld directly on the bolt itself, so the bolt strength will not be reduced.

The above is a brief introduction to anchor bolt pre-embedding.In actual construction, single-plate positioning cannot guarantee good positional accuracy and verticality. When erecting steel columns, we have to ream holes or bend bolts to correct small deviations, which seriously slows down progress. Double locating plates provide far better precision. A steel column can be hoisted into position within just 3 to 5 minutes. The construction efficiency is greatly improved.Therefore, the embedding accuracy directly determines the quality and speed of the next working procedure.

We have tried several methods on our previous projects:

1.Build supports with rebar and fix everything by direct welding. The installation precision is relatively poor.

2.We use construction formwork to make temporary locating plates. After the anchor bolts are firmly secured to the rebar, the formwork plates will be removed.

3.Steel positioning plates are prefabricated in the workshop. A single plate is leveled and set above the concrete surface. After axis positioning, install the anchor bolts. Straighten the bolts and fasten them securely to the rebar. The plates will be dismantled after concrete pouring.

4.Steel locating plates are prefabricated in the factory. We adopt the double-plate system. Set and fix the lower plate first, then install the anchor bolts. Fit another positioning plate on the top of the bolts. This setup strictly controls the relative position, elevation and verticality of anchor bolts. The plates can be removed after concrete pouring and reused repeatedly.

5.For large isolated footings, anchor bolts can be pre-assembled first. We shall build an angle steel frame. Secure all anchor bolts onto the frame as one unit, then hoist the whole assembly onto the foundation. Fasten the frame to the pre-embedded reinforcement on the bedding course or anchor it with post-installed expansion bolts.

6.Methods 4 and 5 achieve the best positional accuracy for anchor bolts.

Anchor bolts often conflict with main rebar. Normally we pry the rebar aside. If the bar cannot be moved, we cut it and reinforce it afterwards.

Double locating plate anchor bolt pre-embedding practical case.

We use double locating plates for temporary positioning of anchor bolts. After concrete pouring, the precision remains very high. The relative position and verticality of bolts barely shift.

First, temporarily weld the lower positioning plate to the bottom stirrups according to the axis line. Then insert the anchor bolt fitted with one nut down through the plate hole from above. Pry aside any interfering rebar with a crowbar so that the bolt can pass smoothly.

After all anchor bolts are inserted in place, adjust the four corner bolts to the design elevation and perform tack welding between the bolts and the lower positioning plate. Screw the nuts to a position 30 mm below the bolt top. Afterwards, install the upper positioning plate onto the four corner bolts and fasten the nuts, while strictly controlling bolt verticality.

Then lift and install the remaining bolts through the corresponding holes of the upper plate, and tighten the upper and lower nuts of the top positioning plate. Finally, fully weld and reinforce the anchor bolts with the positioning plates, as well as the positioning plates with the on-site reinforcement. After overall reinforcement, further welding between the lower section of anchor bolts and the steel bars is no longer required.

Cover the bolt threads with tape to keep them clean and free from concrete contamination.

Performance grade of anchor bolts

Anchor bolts take mainly axial tensile load.That is why we classify them by strength grade.This grade is defined primarily based on tensile strength.

As specified in the standard:Tower connections normally use hot-dip galvanized bolts Grade 4.8, 6.8, 8.8 with matched nuts.Grade 10.9 bolts are optional where feasible.

The 4.8, 6.8 and 8.8 grades are bolt categories defined by their mechanical performance grades.

Detailed explanation:

The figure before the decimal point stands for the tensile strength of the bolt, equal to one hundredth of its tensile strength value.The figure after the decimal point represents the yield ratio, which is the ratio of yield strength to tensile strength of the bolt.

Yield Ratio:The yield ratio refers to the ratio between yield strength and tensile strength of the material.A high yield ratio will lead to brittle failure of the structure. Brittle failure is strictly prohibited in civil engineering. Such failure occurs suddenly without obvious deformation, making it hard to take preventive measures.

When subjected to seismic load, steel will first reach its yield strength and undergo plastic deformation. The visible deformation serves as an early warning sign. People can spot the risk in advance and avoid structural collapse.

Example:

Nominal tensile strength = 800 MPa, yield ratio = 0.8.Therefore, the bolt grade is 8.8.

When talking about anchor bolts, you have surely frequently seen these material designations:

Q235, Q345,

35# high-quality carbon structural steel, 45# high-quality carbon structural steel,

40Cr alloy structural steel and 42CrMo alloy structural steel.

All these classifications are based on raw material grades.

These material grades come from the old standard DL/T 5154-2012, Design Code for Overhead Line Tower Structures.In this specification, anchor bolts were categorized according to steel material rather than strength class.

But under the two newest industry codes:DL/T 5486-2020 and DL/T 1236-2021,the classification rule for anchor bolts has been updated.

Now we categorize anchor bolts based on their performance grade, rather than fixing the steel material grade.We carry out quality inspection following the bolt product standards.

Shapes of Anchor Bolts

An anchor bolt assembly includes the bolt shank, nuts, washers and anchoring parts at the bottom.

As we all know, anchor bolts with bent hook ends deliver superior tensile anchorage.For this reason, various end configurations are adopted to improve the anchoring capacity of foundation bolts.

Classified by the end anchorage configuration,anchor bolts are divided into four types:J-type anchor bolts, T-type anchor bolts,pawl-type anchor bolts and double-thread anchor bolts.

1.J-type anchor bolt

It has a simple structure and is easy to process, mostly applied to light-load towers.The 180° bent hook at the end greatly improves bonding with concrete.

Large-diameter bolts are hard to bend after heating, and it is difficult to guarantee processing precision.

Considering the low bearing capacity and the thermal bending processing limitations of J-type anchor bolts,J-type construction is only recommended for Grade 4.6 anchor bolts.

In view of the strength of Grade 4.6 steel and processing precision,we advise limiting J-type anchor bolts to M36 and smaller diameters.

2.T-type anchor bolt

The T-type anchor bolt is easy to assemble.It can be positioned vertically more conveniently than the J-type, and is suitable for transmission towers with low bearing loads. The T-type anchor bolt adopts an anchor plate at the anchoring end.Four stiffener plates are evenly arranged and welded between the anchor plate and the bolt shank.Welding heat from stiffener plates will reduce the bolt strength to a certain extent.

Considering the low strength, small bearing capacity and welding performance of Grade 4.6 bolts,we recommend choosing the T-type structure. Small-sized T-type anchor bolts are not easy to weld, and welding will weaken the strength of bolts and stiffener plates. Therefore, we recommend adopting T-type anchor bolts of M42 and larger specifications.

3.Pawl-Type Anchor Bolt

For pawl-type anchor bolts, three hook rebars are evenly spaced and welded to the bolt body.

Welding the hook rebars will bring thermal impact and reduce bolt strength.It is used for towers subject to heavy loads. In design, the pawls must stay within the concrete cover and shall not collide with adjacent stirrups. If the gap between the pawls and surrounding steel bars is too small or they come into contact with each other, the orientation of the pawls can be adjusted during installation to prevent collision at the anchorage section when embedding the anchor bolt.

Based on its material strength, bearing capacity and structural form, we recommend Grade 5.6 anchor bolts to adopt the pawl type.

4.Double-thread anchor bolt

The double-end threaded anchor bolt has threads on both ends. It is easy to assemble and features high bearing capacity, so it has a wide range of applications in engineering projects.

The bolt uses a holed anchor plate secured between two nuts, which greatly increases its anti-pullout performance.

Considering the material performance, weldability and load capacity of Grade 6.8 and 8.8 anchor bolts, the double-thread type is recommended.

Considering bolt material, diameter, fabrication difficulty and weldability, the anchorage style and size for each strength grade are shown in the following table.

This table comes from the standard Anchor Bolts and Nuts for Transmission Towers.

Note 1: For transmission towers in areas with an extreme minimum temperature of -30 ℃ and below, Grade 8.8 anchor bolts are recommended.

Note ᵃ: For Grade 8.8 and Grade 10.9 bolts, I-type (double-end threaded) anchor bolts shall be the first option.

How Anchor Bolts Are Named

An anchor bolt’s full product label includes these items:

standard code, bolt type, thread size × total length × thread length, and strength class.

Example: DL/T 1236    L   M36×1200×180-4.6

It means: Standard No.: DL/T 1236

Type: L-type

Thread size: M36

Nominal length: 1200 mm

Thread length: 180 mm

Strength grade: Grade 4.6

Let's break down M36:The letter M stands for metric thread.

36 is the nominal diameter, which is the outer diameter of the bolt, measuring 36 mm.

This is the dimension d shown in the drawing below.

Anchor Bolt Design

As the saying goes: To forge iron, you need a strong hammer.Anchor bolts connect the transmission tower to its foundation.They must resist the uplift force from the tower.That makes the design logic straightforward.

Per the standard requirements:

In short:Total bolt quantity × Effective area of one bolt × Tensile strength must be larger than the tower’s uplift force.

Example: The uplift force on each tower leg is 1400 kN.Can 4 pieces of Grade 4.6 M64 anchor bolts meet this requirement?

The net cross-sectional area of M64 bolt is found to be 2676 mm2.

4×2676 mm2×160 N/mm2=1713 kN>1400 kN

Therefore, four Grade 4.6 M64 anchor bolts satisfy the design requirement.

Using this formula, you can freely adjust the number, strength class and size of anchor bolts.

Normally, 4 anchor bolts are arranged for each tower leg.That is why we often see the marking "4M64".It means four M64 bolts are used on one single tower leg.For a complete tower, the total quantity will be 16 pieces of M64 bolts.

When the tower is large and the uplift force on each tower leg becomes high, four anchor bolts will no longer be sufficient. We can adopt 8, 12 or more bolts accordingly.The base gusset plate shall be specially designed for this arrangement, which is commonly applied to composite steel tube towers.

Equivalent Bolt Conversion for International Standards

Strength Grade Equivalence Table

Metric vs Imperial Size Equivalence (For Capacity Replacement Only, Thread Not Interchangeable)

Effective Tensile Area Ae (mm²)

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