Questions from the Field is a selection of questions NPCA Technical Service engineers received from calls, emails and comments on blog posts or magazine articles posted to Precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/plant-resources/technical-services.
Noura asks: When we calculate the truck HL-93 live load at 5.5m fill height, there is overlap in area, so how should we calculate the load from the wheels when there is an overlap?
NPCA technical experts answer: When there is an overlap, you compute the live load pressure at the overlap area at 5.5m of earth fill and design the structure below for that higher uniform live load. See the diagrams to the right from ASTM C857. Figure 1 shows an overlapping loaded area. Figure 2 shows pictorially the formula for the loaded area for one wheel. Two wheels 1.8m apart would have an overlapping loaded area with 5.5m of earth fill.
Jim asks: We are working on a couple of precast projects in Texas and Arizona. We have a request from the owner to switch from shear walls to precast concrete moment frames. When we performed an initial code review, we were not able to identify the response factor, amplification factor and height limits for a precast concrete moment frame. The code covers shear walls, for sure, and steel moment frames bit silent on PCMF.
We are reaching out in hopes that NPCA has testing reports or guidelines to use in this application. We are building upward of 10 buildings using precast, and this change is creating flexibility needed for the owner. Any help is extremely appreciated.
NPCA technical experts answer: Figure 3 is the design aid table from the Precast/Prestressed Concrete Institute Design Handbook showing the response factors and height limits for precast moment frame structures.