Frequently Asked Questions

Here are some common customer questions and answers:


What is “Tilt Potential,” as shown on the Handling screen?

LECWall estimates the degree of tilt in the panel if all the lines are on rollers and all the rollers are released. It is based on the center of gravity of the panel in relation to the center of all the lift points. This is useful for erection planning. The center of gravity of the panel is listed near the top of the Handling window. Use the C.G. to assist in locating the lift points to provide a more equal distribution of lift forces. Top of panel is shown on the left, so lifters could be positioned to induce a clockwise tilt, which would bring the top of panel up as the panel is lifted from the trailer.



While running LECWall, I came across two questions about how the program considers prestressing forces:

1. While doing a P-delta analysis, which load cases consider the axial force due to prestressing?

2. What load factor is used for prestressing force?

1. Axial force due to prestress is present for all load cases. It does not contribute to P-Delta effects directly. If prestress is eccentric and induces bow, then Delta will increase, causing a net greater P-Delta due to other applied gravity loads. Even so, the prestress force always follows the curve of the panel and is internal to the panel, so does not contribute any P-Delta force of its own.

2. No load factor is used for the prestress force (Load Factor = 1).



Does LECWall check if a shear connector can provide 100% composite action?

LECWall displays the shear along the panel at 50 points. It is up to the engineer-user to determine how many shear connectors to use along the span to achieve the desired composite action. The individual connector shear strength could be obtained through testing or manufacturers’ recommendations.



When checking handling I get a message in red text underneath the graphs that says "Note: Reinf ratio < ACI min. - capacity reduced 25%". What part of ACI does this reference?

This warning is for minimum flexure reinforcement. The relevant ACI Code section is 318-08, Section 10.5, specifically 10.5.3. 318-11 is the same. It states that As needs to be increased by 1/3 if minimum reinforcing for flexure is not provided. That is equivalent to a 25% reduction in capacity.

Also, please note that the reinforcing input unit is bar diameter, not area. If bar area is input by mistake, that could trigger the under-reinforced message.

 


When running a service load case, sometimes stresses show as red on the diagram even though the section is uncracked at that location. Why is that?

For service load cases, LECWall uses the “Allowable Service Tension Stress Coefficient” specified on the “Load Case Coefficients” screen (attached).  This defaults to 5 sqrt F’c for mild reinforced members.  So, although the member is not cracked, tension stresses exceed that recommended by PCI for serviceability when exposed to view and weather (see PCI Handbook, 7th Ed. Example 5.2.2.1).



How does LECWall calculate the cracking moment for a wall panel? Are any load factors included?

Self-weight, applied axial load and prestress force all increase the cracking moment.  Load factors are also applied, consistent with the load case being considered.  Sometimes mild steel needs to be added to prestressed members to satisfy the ACI 1.2Mcr requirement. The requirement initially applies at the location most likely to crack first.

 


How should I specify lifter locations for the situation where there are double rollers for one of the lines without a spreader bar?

LECWall assumes two lines of support to maintain static determinacy. The pick point locations and gaps for this case can be modeled as shown in the illustration below. The left pick point A is at the top of panel. The right pick point E is as shown, measured from the right edge (panel bottom). The right gap should be the distance from the left roller centerline C to E, not from the actual lifter locations, B and D. This is conservative, but necessary to correctly model the forces in the left and right lift lines.


 


LECWall gives me this message: "ACI 318-14 Maximum slenderness ratio of 1.4 may be exceeded."

This requirement can be found in Section 6.2.6 of ACI 318-14.  The multistory (.W12) version of LECWall can’t check this directly so it counts the number of P-Delta iterations instead.  The message displays if iterations are greater than 4, signifying a slender member which may have significant secondary (P-Delta) moments.

There are two ways to check the member if you are receiving this message.  One way is to calculate the moment at mid-height manually without any P-Delta effects and compare it to the LECWall moment.  If the LECWall moment is less than 1.4 times the manual result, then it’s OK.  The second is to re-input the member as a single-story member (.WAL).  This simpler analysis checks the 1.4 requirement directly and will give you a message if it is exceeded.



LECPres gives me errors when I try to run it in Windows 7/8/10.

LECPres needs to be run in Compatibility Mode.  Right click on the program launch icon, select “Properties,” then the “Compatibility” tab.  Check the box to run the program in “Compatibility Mode” for Windows XP, Service Pack 3.