GENERAL APPROACHES TO FABRICATION AND ERECTION OF BRIDGE STEELWORKS BASICS AND TUTORIALS

FABRICATION AND ERECTION OF BRIDGE STEELWORKS GENERAL APPROACHES
What Are The General Approaches To Fabrication and Erection Of Bridge Steelworks

The objective in steel bridge construction is to fabricate and erect the structure so that it will have the geometry and stressing designated on the design plans, under full dead load at normal temperature.

This geometry is known as the geometric outline.

In the case of steel bridges there have been, over the decades, two general procedures for achieving this objective:

1. The “field adjustment” procedure — Carry out a continuing program of steelwork surveys and measurements in the field as erection progresses, in an attempt to discover fabrication and erection deficiencies; and perform continuing steelwork adjustments in an effort to compensate for such deficiencies and for errors in span baselines and pier elevations.

2. The “shop control” procedure — Place total reliance on first-order surveying of span baselines and pier elevations, and on accurate steelwork fabrication and erection augmented by meticulous construction engineering; and proceed with erection without any field adjustments, on the basis that the resulting bridge deadload geometry and stressing will be as good as can possibly be achieved.

Bridge designers have a strong tendency to overestimate the capability of field forces to accomplish accurate measurements and effective adjustments of the partially erected structure, and at the same time they tend to underestimate the positive effects of precise steel bridgework fabrication and erection.

As a result, we continue to find contract drawings for major steel bridges that call for field evolutions such as the following:

1. Continuous trusses and girders

— At the designated stages, measure or “weigh” the reactions on each pier, compare them with calculated theoretical values, and add or remove bearing-shoe shims to bring measured values into agreement with calculated values.

2. Arch bridges

— With the arch ribs erected to midspan and only the short, closing “crown sections” not yet in place, measure thrust and moment at the crown, compare them with calculated theoretical values, and then adjust the shape of the closing sections to correct for errors in span-length measurements and in bearing-surface angles at skewback supports, along with accumulated fabrication and erection errors.

3. Suspension bridges

— Following erection of the first cable wire or strand across the spans from anchorage to anchorage, survey its sag in each span and adjust these sags to agree with calculated theoretical values.

4. Arch bridges and suspension bridges — Carry out a deck-profile survey along each side of the bridge under the steel-load-only condition, compare survey results with the theoretical profile, and shim the suspender sockets so as to render the bridge floor beams level in the completed structure.

5. Cable-stayed bridges

— At each deck-steelwork erection stage, adjust tensions in the newly erected cable stays so as to bring the surveyed deck profile and measured stay tensions into agreement with calculated theoretical data.

There are two prime obstacles to the success of “field adjustment” procedures of whatever type: (1) field determination of the actual geometric and stress conditions of the partially erected structure and its components will not necessarily be definitive, and (2) calculation of the corresponding “proper” or “target” theoretical geometric and stress conditions will most likely prove to be less than authoritative.

PROPERTIES OF HARDENED CONCRETE BASICS AND TUTORIALS

HARDENED CONCRETE BASIC PROPERTIES
What Are The Properties of Hardened Concrete?

Fully cured, hardened concrete must be strong enough to withstand the structural and service loads which will be applied to it and must be durable enough to withstand the environmental exposure for which it is intended. When concrete is made with high-quality materials and is properly proportioned, mixed, handled, placed, and finished, it is one of the strongest and most durable of building materials.

When we refer to concrete strength, we are generally talking about compressive strength which is measured in pounds per square inch (psi). Concrete is strong in compression but relatively weak in tension and bending.

It takes a great deal of force to crush concrete, but very little force to pull it apart or cause bending cracks (Figure 2-3). Compressive strength is determined primarily by the amount of cement used but is also affected by the ratio of water to cement, as well as proper mixing, placing, and curing.

Tensile strength usually ranges from 7 or 8% of compressive strength in high-strength mixes to 11 or 12% in low-strength mixes. Both tensile strength and flexural bending strength can be increased by adding steel or fiber reinforcement.

Structural engineers establish required compressive strengths for various building elements based on an analysis of the loads which will be applied and the soil conditions at the project site. Actual compressive strength is verified by testing samples in a laboratory using standardized equipment and procedures.

On commercial projects, numerous samples are tested throughout construction to verify that the concrete being put into place actually has the specified strength. Laboratory testing is not often required in residential work, except perhaps on large, high-end projects or on projects with difficult sites where special foundation designs make concrete strength critical.

For most residential projects, required concrete strength will be in the range of 2,500 to 4,000 psi, depending on the intended use (Figure 2-4). A concrete that is stronger than necessary for its intended use is not economical, and one that is not strong enough can be dangerous.

The primary factors affecting concrete compressive strength are the cement content, the ratio of water to cement, and the adequacy and extent of hydration and curing, all of which are discussed later in this chapter.

Durability might be defined as the ability to maintain satisfactory performance over an extended service life. Satisfactory performance is related to intended use. Concrete that will be walked or driven on must be abrasion resistant so that it doesn’t wear away.

Concrete that will be exposed on the outside of a building must be weather resistant so that it doesn’t deteriorate from repeated freezing and thawing. Concrete in which steel reinforcement is embedded must resist excessive moisture absorption in order to protect the metal from corrosion.

Natural wear and weathering will cause some change in the appearance of concrete over time, but in general, durability also includes the maintenance of aesthetic as well as functional characteristics. Just as concrete mix designs can be adjusted to produce a variety of strengths, appropriate concrete ingredients, mix proportions, and finishes can and should be adjusted on the basis of required durability.