by Josh Taylor
Solutions Discussed: Structural design of concrete-framed buildings
Products Discussed: RAM Structural System
This article documents a few instances, or frequently asked questions, in RAM Concrete where the design results that have been produced may not look right at first glance, but nonetheless are proper interpretations of ACI 318. There is one case each presented for the beam, column, and wall modules. If you've ever seen one of these situations during a session in RAM Concrete, hopefully the explanations below will shed some light on the matter.
Frequently Asked Question 1 - RAM Concrete Column
Q: I have a rectangular concrete column that experiences little shear force. I am using the Ordinary Concrete Frame option in RAM Concrete Column. Why, for certain columns, does RAM Concrete place a zone of tighter reinforcement at the high end of the column when the balance spacing will work fine for strength requirements?
Figure 1 - Zone of tighter shear reinforcement at high end of column.
A: The most common reason for RAM Concrete choosing a zone of tighter reinforcement at the top of the column is to fulfill Section 11.11.2 of ACI 318-05. The only condition under which this section does not require fulfillment is if the column is not part of the lateral load resisting system (in the case of RAM Structural System, is assigned as a gravity member), and the column is restrained on all four sides by beams or columns of roughly equal depth. Otherwise this check is considered by RAM Concrete and the reinforcement is laid out appropriately. If the tie requirements due to all other strength and spacing checks is light, Section 11.11.2 many control the design at the top of the column and thus a separate zone is produced with a tighter tie spacing.
Frequently Asked Question 2 - RAM Concrete Beam
Q: I am designing the beams of a special moment-resisting frame in RAM Concrete. I notice cases where RAM Concrete Beam selects tighter stirrup spacing over the middle third of the beam span, rather than tighter spacing at the outer thirds of the span. Why is this?
Figure 2 - Zone of tighter stirrup spacing at interior of beam span in RAM Concrete Beam
A: ACI 318-05, Section 21.3.2.3 requires that lap splices of flexural reinforcement in special moment frames be enclosed with hoop reinforcement over the lap length. The spacing of the hoops is not to exceed the smaller of d/4 and 4 in. Often this requirement exceeds the strength and prescriptive requirements at the outer thirds of the beam, thus a tighter spacing will be provided at the interior.
Frequently Asked Question 3 - RAM Concrete Shear Wall
Q: I have a multi-story wall stack with a doorway at each level. The reinforcement provided by RAM Concrete is larger than I would expect to see in the wall piers, and capacity is generally well in excess of demand. Why is this?
A: RAM Concrete checks the reinforcement required at all section cuts in a wall panel and attempts to select one bar pattern that will suffice for the entire wall panel. Often times the headers represent a critical design case that requires substantially more reinforcement than the adjacent wall piers. Thus, this reinforcement will be applied over the entire wall panel. However, there are ways to achieve a more economical design. See the blog article entitled These Wall Headers are Killing Me! What Can I Do? at http://communities.bentley.com/Other/Old_Site_Member_Blogs/Bentley_Employees/b/josh_taylors_blog/default.aspx for a detailed discussion.