10.6 Erection and Equipment Loads on Permanent Structures

Introduction to Construction and Erection Loads

A fundamental challenge in construction engineering is managing loads imposed on permanent structures during the erection phase. Structural design codes (such as ASCE 7) ensure completed buildings can safely support final service loads. However, during construction, the partially completed structure is often subjected to its most critical load cases. These temporary loads arise from staging materials (e.g., structural steel bundles, drywall pallets, concrete block stacks), operating heavy machinery (e.g., crawler cranes, concrete pumps, scissor lifts) on elevated decks, and lifting structural elements during assembly. The construction engineer must analyze these temporary loads to prevent localized damage, serviceability failures, or catastrophic collapses.

Outrigger Loads and Soil/Slab Bearing Pressure

When heavy mobile cranes or concrete pump trucks operate on soil or elevated concrete slabs, their total weight—plus the payload—is concentrated through outrigger pads. The bearing pressure ($P_b$) exerted by a single outrigger is: $P_b = \frac{P_{outrigger}}{A_{mat}}$ where $P_{outrigger}$ is the maximum outrigger force (lbs) and $A_{mat}$ is the area of the supporting crane mat or timber cribbing ($\text{sq ft}$).

On soil, excessive outrigger pressure can cause bearing capacity failure, tilting the crane. On concrete slabs, it can trigger punching shear, where the outrigger punches directly through the slab. To mitigate this, contractors use thick timber mats or steel plates to distribute the load over a larger tributary area, reducing the bearing pressure to within the allowable limits of the soil or slab.

Rigging Mechanics and Sling Tension

Erection operations rely heavily on rigging assemblies to lift and place heavy precast concrete members, steel trusses, and mechanical equipment. Analyzing the forces in rigging hardware is critical for crane safety.

Sling Angle Tension Calculations

When a load is lifted using a multi-leg sling, the tension in each leg is not simply the weight of the load divided by the number of legs. The tension increases as the sling angle relative to the horizontal plane ($\theta$) decreases. For a symmetrical two-leg sling system supporting a load of weight $W$: $T = \frac{W}{2 \sin(\theta)}$ where:

• $T$ is the tension in each sling leg (lbs or kips).
• $W$ is the total weight of the load, including rigging gear (lbs or kips).
• $\theta$ is the angle between the sling leg and the horizontal plane of the load.

As $\theta$ decreases, $W_{design}$ must also be adjusted. As $\theta$ approaches zero, the tension $T$ increases exponentially.

• At $\theta = 90^\circ$ (vertical lift), the tension in each leg is exactly $0.5 W$.
• At $\theta = 30^\circ$, $\sin(30^\circ) = 0.5$, and the tension in each leg equals $W$. Each leg must carry the entire weight of the load.
• At $\theta = 20^\circ$, $\sin(20^\circ) = 0.342$, and the tension is approximately $1.46 W$, which is nearly three times the vertical load.

For this reason, rigging angles below $30^\circ$ are extremely dangerous and generally prohibited by OSHA standards. Furthermore, low sling angles introduce high horizontal compressive forces ($H = T \cos(\theta)$) into the lifted member, which can buckle long precast concrete girders or steel trusses.

Dynamic Impact Factors

Static load calculations do not account for the accelerations and decelerations of hoisting. Sudden movements (snatching a load or slamming a brake) introduce dynamic forces. To design safely, engineers apply a Dynamic Impact Factor ($I$) to the static load, typically ranging from $1.25\text{ to }2.0$: $W_{design} = W \times I$

Worked Engineering Example: Outrigger Matting and Rigging

Problem Scenario: A mobile crane is lifting a precast concrete wall panel on an elevated concrete deck.

1. The crane's maximum outrigger reaction is $96,000\text{ lbs}$. The structural engineer limits the allowable bearing pressure on the slab to $3,000\text{ psf}$ to prevent punching shear. Determine the minimum side dimension of a square timber mat required beneath the outrigger pad.
2. The precast wall panel weighs $16,000\text{ lbs}$ and is lifted using a symmetrical two-leg sling. The rigging configuration dictates a horizontal sling angle of $\theta = 45^\circ$. Calculate the tension in each sling leg and compare it to a vertical lift.

Part 1: Mat Dimension Calculation

• Required Mat Area ($A_{mat}$): $A_{mat} = \frac{P_{outrigger}}{P_{allow}} = \frac{96,000\text{ lbs}}{3,000\text{ psf}} = 32.0\text{ sq ft}$
• For a square mat ($B \times B$): $B = \sqrt{A_{mat}} = \sqrt{32.0\text{ sq ft}} \approx 5.66\text{ feet}$ Answer: The contractor must use a square mat with a minimum side dimension of $5.7\text{ feet}$ (typically a standard $6\text{ ft} \times 6\text{ ft}$ mat).

Part 2: Sling Tension Calculation

• Apply the sling tension formula for a two-leg sling: $T = \frac{W}{2 \sin(\theta)} = \frac{16,000\text{ lbs}}{2 \sin(45^\circ)} = \frac{16,000}{2 \times 0.7071} = \frac{16,000}{1.414} \approx 11,314\text{ lbs}$
• Compare to vertical lift ($\theta = 90^\circ$): $T_{vertical} = \frac{16,000}{2 \sin(90^\circ)} = 8,000\text{ lbs}$ Answer: The tension in each sling leg is $11,314\text{ lbs}$, which is a $41.4%$ increase in tension compared to the $8,000\text{ lbs}$ experienced during a vertical lift.

Critical Field Traps in Erection Operations

• Neglecting Rigging Weight: A common trap is failing to include the weight of the spreader beam, hooks, and shackles in the total lift weight ($W$), which can lead to overloading the rigging or crane.
• Outrigger Placement Hazards: Placing outriggers over underground utilities, vaulted voids, or near the edge of excavation trenches. The lateral pressure from the outrigger can collapse trench shoring or punch through utility vaults.
• Underestimating Lateral Tension Forces: Lifting a flexible member (like a steel plate or truss) with a low sling angle generates massive horizontal compression in the member. Without a spreader bar, this compression can cause the member to buckle during the lift.